That's wonderful! Toddlers almost always act in ways that in an adult would be considered OCD. Our current culture is way too quick to label and even pathologize normal behavior. (In this case, a *better* than normal behavior.)
Moreover, if your child is receptive to learning letters at this stage, you're not pushing him, you're following the child's lead. The kid's hungry - gotta feed him! Talk to him, sing to him, play with him, READ with him, make his world rich with print and opportunities to learn as fast as he wants.
No one is surprised when a toddler can name 50 different animals and the sounds they make, or the names and "superpowers" of a myriad of cartoon superheroes. So being able to recognize letters and the related sounds isn't really such a stretch. Take advantage of this time while he's still interested. But ALWAYS make it FUN! The more literacy skills your child can pick up at this stage, when he is interested (even obsessed) and enjoying the activities, the easier reading will be. Very young children CAN learn a lot of reading skills, when the environment supports (but doesn't push) it.
As long as the child is enjoying the interaction with you, and he's having fun, it's fine to teach quantum mechanics (although I admit that would require some very clever parenting.)
If you have some of those foam bath letters that stick to the side of the tub, make a game of collecting all of them before the bath, sound out real or nonsense words during bath time. My kids love to string together longer and longer "words" and then I have to try and sound out "giwplo" or "hdpsu" - the crazier the word, and the more difficulty I have trying to sound it out, the more they laugh.
PLAY with letters, incorporate them into all kinds of games, every day. And introduce lowercase letters now - most of written language is lowercase, but most early introductions for children are all capitals. What's UP with that?
Sit your child on your lap and and read, making sure he can see the words, and use your finger to follow the words. Poetry is great at this age, because they start getting used to hearing rhymes, and anticipating words (especially if you pause for just a second before the rhyming word, the brain automatically starts thinking of what will come next.) Picture books will get him interested in the beauty of books. You might want to start finding some books with very simple words in large print, although generally these aren't as interesting and doing too much of those could make him lose interest. ("Yo! Yes?" is *wonderful*, though, as is the series "Brand New Readers" - my kids are quickly bored by the "Bob" books.)
When you come across a page with a picture of something and the word for it, make a point to point to the object and the word a couple of times ("Here is the honey, and here is the *word* honey") - later you can ask your child to point to them.) Make reading together an important part of every day. While it's a wonderful way to ease your child off to sleep, it's also good to do some when they are awake and alert.
Make it enjoyable and spontaneous - be silly and creative. (Unless you can make it fun and intersesting, I wouldn't particularly recommend flash cards, for instance, although every child is different.) Find ways to assemble words from letters, play with words, point out signs when driving, or when shopping. My toddler had a big breakthrough as we drove through a park - we pointed out the sign "STOP" and then we'd have to stop. Then we pointed out the sign "BUMP" and then we'd go over the bump. We made a game of that each time we visited the park, and then one day, he did the sounding-out strategy for both signs. He was SO excited when he figured that out and wanted to go home and sound out words for the next half hour!
Make labels for all kinds of things, using sticky notes, or even better, a real label-maker. (Sooner or later, you'll need one for labaling things for daycare, school, and camp, so just go to the office supply now.)
Make a game of putting labels on all kinds of things around the house - door, table, shower, spoon, Mom, Dad, cat. (The sticky note stayed on our cat for about 5 seconds, but the giggles made it memorable.)
Show him his own name, and make a label. Buy or make a personalized cup or water bottle with his name on it. Play rhyming games, spoken and written, sing songs, etc. So many children can memorize a song without even trying, so capitalize on this in creative and fun ways, like playing Old MacDonald and using labeled pictures as cues, and then once he has it down, try it with words for the cues "cow" and "moo", "duck" and "quack".
Then try the mixed-up mommy, who gets all the sounds mixed up and watch him laugh as he corrects you.
My toddler *loved* to play what we called the "big words game" where we said a word and he repeated it, and the words got harder and harder. (He had really great diction, and he LOVED this game). The catch was if we said "Constantinople" he was supposed to reply "Istanbul!" - it was a little like a verbal form of "Simon Says".
Make it fun. Be creative. Be consistent. And be patient. Follow the child's lead. If you accidentally push a little too hard, your kid probably WILL let you know it. Many children *do* learn to read at a very early age, (my husband was able to recognize many words before his second birthday) and it isn't from parents foisting it one them, but supporting the child's own interest.
Oh, and don't try to incorporate writing this young because the motor skills aren't there. Older children learning to read often learn writing at the same time, and they seem to reinforce each other at that age. But when VERY young children learn to read, it's too much to expect writing as well, and it would probably frustrate.
(The Hoagies website is filled with stories of children that were very early readers. Very inspiring!)