E.T.
Do you think your son is not chewing/eating certain foods because he'd rather get things like Cheetos? If that's the case - stop offering treats. Simply provide healthy food and that's it. If he's hungry, he'll eat.
But... if you think he might be having a problem with certain textures, then talk to your pediatrician about getting a referral to a speech pathologist/feeding therapist. At first I thought this was what your post was about - that your son is having problems chewing certain foods. A therapist can do wonders for a kid with this problem. She will make eating fun, play games with food, show him how to properly chew and swallow. You'd be amazed at how many kids refuse certain foods and textures because their tongues get in the way or they swallow funny. Getting him help now will eliminate years of frustration for you both.
But... if what you're talking about is that your son is becoming a picky eater and would prefer junk to real food, that's something you can solve on your own. Don't give him Cheetos, candy, cookies, or whatever it is he wants more. Above all - stop fighting the battle. He is able to control what he eats. It's just about one of the only things he *can* control at this age. If you make eating a struggle, you will lose. Remember that your job is to provide the food, his is to eat it. If he doesn't do his job, that's on him. Not you.