S. -
Well, I'm not a doctor, but I don't agree with your doc. I think toddlerhood is a very stubborn "control" phase, and it sounds to me like you are in a control battle with your son that is centered around food. My son is in his 2's, and food is a big issue, but my doc told me to make it a NON-issue and be very laid back about it, so my son begins to understand that there are no control issues associated with eating. This has worked amazingly well. My doc was also very clear that kids will get what they need, even if you are positive they aren't getting enough, etc. To be on the safe side, I started giving my son vitamins and he LOVES them - they are a special treat he begs for.
I do limit bad snack foods - like he doesn't ever have access to potato chips, and I usually don't order french fries. But when we have meals, I put a little bit of everything we're eating on the plate, and I make sure he has something he'll eat, hot dog, whatever, and some days he's tries everything there is, some days, only the hot dog, some days nothing. He tells me when he's done, I let him get up from the table immediately. If for whatever reason I really want him to eat a bit more, I offer him my fork (the salad size). He likes to use my "big boy" fork to spear his food, and will usually take a few more bites this way.
One other trick I know is to try eating with another family that has children just slightly older. My toddler loves his older nephews and imitates them - inlcuding eating what they eat.
In any case, once my doc told me to let go of the controlling aspect of it, I've had much more success in introducing new foods. It is "no big deal" if he tries it, or doesn't. This is the time period when toddlers really want to be in control, so maybe your issue isn't about food at all. Something to think about...but I personally don't agree with witholding food - I think that starts the cycle of food obsession that could be problematic on down the road.
Good luck!
S.