J.-
Ellen is right! As hard as it may be to hear- much less DO- giving in to your son's tantrums will only teach him that bad behavior will get him what he wants. And the longer you hold out before giving in, the longer he'll be willing to push next time, because he knows you'll give in eventually.
I'm very fortunate. My daughter has, for the most part, always eaten healthfully willingly. I have dealt, though, with new resistance to foods she used to like and reluctance to try anything new. With that, we made a deal. My part was to put a lot of effort into making good-for-her taste really good. I like to cook, so that part is fun for me. Also, whatever I know she likes that IS healthy, I will fix infinately while trying out new stuff (LOTS of veggies and dip). Her part is that, whatever I fix or she encounters anywhere else, she must eat 2 kid-sized bites of, then no more unless she wants to. Her dad introduced the notion that you grow all new taste buds every 6 months, so you never know what you might suddenly like. I can't verify that, but it works, so who cares.
This has been VERY successful. She always eats healthy stuff she enjoys and we've added many things to that list. In mommyhood, to me it's important to pick your battles carefully. Your child's health is worth it. Good luck!
C.