17 Month Old Doesn't

Yes.

I have 4 kids. They ALL did things differently.

#1: born 3 days early, got teeth at 8 months, crawled at 7 months, walked at 13 months, spoke complete clear sentences at 18 months.

#2: born 4.5 weeks late, got teeth at 4 months, crawled at 8 months, walked at 10 months, spoke clearly at 3 years old.

#3: born 12 days late, got teeth at 12 months, crawled at 6 months, walked at 9 months, spoke clear sentences at 14 months....and hasn't stopped yet! (She's our only girl.)

#4: born 6 days late, got teeth at 10 months, crawled at 7 months, finally walked at 17 months (1 week ago)....and currently at almost 18 months, he grunts. He understands everything. He will crawl off the bed and take his own wrapped up diaper to the trash, but he will not say actual words. He will shake NO when he REALLY doesn't want something and now 2 days ago started signing for "more." He's never shaken his head YES. I ask him to clean up his toys and he does.

Yep!!!

And remember: Nodding or shaking is cultural/learned.

Our culture nods for yes, and shakes for no.
Some cultures nod for no, and shake for yes.

Toddlers often get this backwards... Especially as shaking their heads violently is more "fun"/ what they do when happy (full body happy wiggle), and nodding is evocative of screaming (try it: head down, then lift to "howl", down to breathe, then lift again to howl).

So even when he does, expect him to mix up the body language cue from time to time.

Very normal. My 20 mos old granddaughter does not shake, nod, or say yes or no. She's not even saying many words. As your son did, she will run to the item or activity that she wants. That shows she understands what you said. Babies learn to understand before they learn to talk.

Yep! With my oldest I usually said, "Do you want lunch? If you want lunch, say, 'Yes!'" Worked for him! Did not work for my youngest. Funny how kids are different.

If it means a lot to you, I would work on it while you're both sitting at the table. You could show him toys and say things like, "Is this your car? No (shaking your head), this is your teddy bear. Is this your car? Yes (nodding your head), it is your car!" Really exaggerate the motion. He'll pick up on it.