Boy do i hear you. We are in the same place. All i can say it to keep trying. (Also, i can recommend putting pureed vegetables in pancakes or mac&cheese, which is something i've had luck with.)
If your doctor isn't concerned about your sons health, though, it seems like this just takes time. Ask the doctor at the next appt to be sure, but, i imagine your son is ok. Your stress, while i get it, probably isn't helping, and pushing at meal times probably isn't helping. Its really hard to let it go. I've been doing a lot of research on the issue, and this is what pretty much everything says.
Keep offering things, and see if you can find something that works. Remember that yogurt and cheese do have protein, and fruits do have vitamins, so its not all dire. You could try hummus, pizza (strangely, my daughter likes this, and we like that it has tomatoes), vegetables picked up raw (like peas), lunch meat, things baked into tasty muffins or cakes, hot dogs, dinosaur nuggets, etc.
One of my friends also suggested having my daughter eat with another kid her age who eats well. Doesn't seem to work for us, but maybe for you? Another friend suggested using an opposite psychology - make a huge show of enjoying something, but deliberately don't offer it to your kid. Put it within reach, and then see if your kid will take some while your back is turned.