I can think of at least five ways you could respond. I don't know you, your coworker, your situation, or the company at all, so of course these are all just generic:
1. Thank him for his suggestions/comments, incorporate them into your work product, let the rest of your team know of his contribution, and assign him a little more responsibility -- for example, give him more to "nitpick" for you.
2. If you find some of his suggestions/comments useful and others not so much, treat the more useful ones as in 1. above, and the less useful ones as in 3. below.
3. For any suggestions/comments that you're sure won't work as well as what you'd proposed, thank him as always, then go a little out of your way to explain to him why you think your version is going to work better. He's still free to make suggestions, of course, but you're raising the bar. In order for you to pay serious attention, he's going to have to improve the quality of his criticisms.
4. For any of his suggestions where you recognize that your version could have been better but so could his, thank him as always. Let him know that there's still room for improvement and ask him to suggest yet another version.
If you can't carry out one of the above convincingly then you may in effect be defaulting to this last one:
5. Make it clear to him that he's junior to you, you have more experience than he has, you find his nitpicking annoying, and he needs to stop immediately if not sooner -- or else! As I said, I don't know you or him or the company but I give this strategy about a 5% chance of working out the way you'd like. More likely, he's going to jump to some silly conclusions like that at least one of you might feel insecure; that at least one of you finds the other a threat; that the way to neutralize you and/or keep you at a safe distance is by nitpicking your work every chance he gets; and that at worst, how you react when he succeeds in pushing your buttons is sure to make for some great gossip among him and his friends both inside and outside the company.