ugh! Does your doctor even have children?
Firstly, what percentile is your child in? I am always wary of other people who comment on a child's weight/age. I was very very skinny as a child and I have turned out fine. If your LOs health is otherwise fine then really, the weight will follow.
feeding has to be the most frustrating job. I totally feel your pain, its so annoying and boring! It sounds like you've followed your doc's advice and tried to enforce eating times. Which is why he now hates the high chair. It also sounds like you have a super active LO on your hands who probably just does not want to sit still (lets face it, if you'd just learned all these new things about your body why sit down?) I've got a few bits of advice that I hope helps in some way to relieve your frustration...take what you want, leave what you don't. I really hope it gets better, cause I know how you feel!
* Check out the stickies on portions for toddlers. Often we think they should be eating more but a toddler portion is much less than ours.
* Try to pretend that it doesn't bother you if he eats or not. If he senses that it gets you hot and bothered then he will react to that. Suppress your anger and take a 'whatever' attitude. Letting him see your frustration or making a big deal of it is just adding more pressure.
* You could try instead of the high chair a little toddler's chair at the table or your lap until he forgets how much he hates the high chair. I do this with DS sometimes when he is really distracted. Some people will say that's a bad habit but he won't do it forever and the main thing is to ease your frustration and get him eating.
* It may be that he has a quick metabolism (could account for his skinniness) and is the type of person who needs to eat little but often. You could try putting plates/bowls of snacks around him while he is playing, let him know its there if he wants it and keep reminding him occasionally.
*I also pretend to eat my son's food and make lots of yummy noises while doing it. While doing this I pretend to ignore him, ie. so engrossed in the food...often he gets curious at my antics and wanders over to see and will have some bites.
* When you eat your own food be sure to make positive comments about how much you enjoy it and how tasty each thing is. Don't worry if it looks like he doesn't notice, these kids don't miss a trick, he's taking it all in
* A fave at our place is chicken patties. Mince a chicken breast, grate some veg (carrot, zucchini, spinach, whatever) add some bread crumbs, egg and any herbs you like, mix together well, dip in bread crumbs, place on a tray cover, freeze. when frozen transfer to freezer bag or container. To cook, defrost and lightly pan fry. You can do this finger size as a meal or snack, tasty and nutritious. Use dark chicken meat if you want extra calories/good fats.
* and lastly, try not to take it to heart. It is a phase, it will pass and he won't be like this forever
hugs x