My 10 year old hardly eats

My son was diagnosed almost 2 years ago. He is almost 11 years old and due to start mainstream high school in September.  He is only 135cm tall.  His height and weight are monitored when he attends appointments every 6 months or so.  I took him for his most recent appointment and he appears to have lost weight unless they screwed up last time.  (His height chart said that he had grown 12cm in 6 months!!).  

The problem is, he hardly eats anything.  Chicken nuggets, sausages (picked apart while he is eating), fish/fish fingers, meatballs, roast dinner (no veg and only chicken).  He will only have cheese spread in a sandwich.  School check every day to make sure he is eating his lunch which he does but to be honest I don't think I put enough in his lunchbox because if I put anything other than a sandwich, a frube and a couple of small cake bars he will leave everything else. 

We are constantly trying to get him to try different foods but as soon as it hits his lips he is pulling his face.  I tried to get him to eat a jelly the other and he gagged on it.

Can anyone help with how I can persuade him to try other things or help in any other way.  We are at our wits end.

Thanks guys