I think you will know your horse better than anyone. He will give you signs if it is too much for him, backing off the jump, knocking rails, stops, run-outs, etc. The key is to slowly work up to it. Add higher jumps here and there and get him used to it before adding full courses. He will tell you when its his stopping point or if he needs to take it slower.
I love that you say you are mainly doing flat-work. It is the key to everything your horse needs, and the more in shape, balanced, using his body correctly, etc. he is, the easier it will be for him to jump higher or jump period.
Adding cavelletti and poles to your flat-work adds a element that helps make their jumping sessions easier and their flat-work sessions more fun.
I would talk to your vet and trainer together to discuss his limits and your options, good luck! :)