I cut my son's hair (now 3) since he was a toddler like your son's age. I sit him in the booster seat and distract him with a little show and a treat (like a lollipop or m&m's or something). I also cut my husbands hair so my son got to see that done many times so he was excited when it was his turn.
As for the actual technique, that's harder to explain. I call his hairstyle "a little boy regular" but I don't know if that's really the name or not - growing up my dad always called it that to my brothers. I use the clippers on the bottom of his hair and hairstyling scissors on the top and around the ears. I'll break this into the three basic steps that I use:
1. I first buzz the bottom and sides, even around the ears, with clippers using the 6mm attachment and then go back over the very bottom by his neck with a 3mm and taper it off lightly as I go up (to "fade it" into the rest of the back).
2. For the top, I generally pick up his hair with my fingers (like you see hairstylists typically do when they slide a section through their fingers) and bring it up about 3/4" or so then cut. First I would go from front to back across the whole top of his head and then repeat going the opposite way(side to side) to make sure I got everything to be about the same. I don't bother using a comb, just keep picking it up with my fingers until I get it all, just takes a few minutes.
3. Lastly I got back over the sides to blend the top of the hair with the buzzed sides. To do this, I take the comb and bring it vertically along the side of his head. So basically, I take the comb and lightly hold it against the side of his head so it's flush with his temple sticks up into the air (so it basically goes from about his ear and sticks up above his head). As you bring the brush straight back towards the back of his head, cut any hair that sticks out (it will mostly be hair from the top) from the teeth of the comb. Depending on your sons hair, you may find it easier to start from the back.
You can probably skip step 3 depending on what style you're looking for, how comfortabe you're feeling or how well your son is doing. I didn't add on step 3 until he was used to it a few times but it really doesn't take more than a few minutes. And if you see any long hairs but the ears, just push the top of the ear gently down and cut around to naturally follow the ear line.
Hope this helps. It gets easier and faster the more times you do it. If you've nervous or have never really seen this done before, you may want to take him to the barber or hairdresser the first time to watch and ask questions. Another idea would be to watch a few clips on youtube. You can just enter "how to cut hair" or something in the search box. Good luck!!
Good luck! I think it's something worth learning because it saves so much money. However, there are some places that charge very little (like the place in my town apparently only charges as much as the child's age).