I think that you should expect help from your son's teacher to manage his trumpet playing whilst in braces. I am not a brass player but my understanding is that the thing which can suffer in braces is the top register. Your son's teacher really should be aware of the problems and either be patient until they come off or help him to work round the problem.
I had one oboe pupil who simply couldn't cope with the wires in his mouth (but he was having massive amounts of metalwork done in his mouth) but I've managed to keep many, many others going, recognising the limitations such things bring for the period you are putting up with the braces. One further thought though is that the mouth shape will change as the teeth move so there is often some adjusting to be done to the embouchure when the wires are removed.
It is always such a liberating time when they come off

. Almost universally, my pupils usually arrive (especially if they haven't told me they're getting the brace removed) at their lesson grinning from ear to ear and then we start to repair and reshape the embouchure which has been changed by wearing braces and then a new shape to the mouth.
So maybe difficult now but exciting times to come - but I do feel your son's teacher really should help not criticise in this period. I'm sorry to hear he's not been well but if he's well enough now to teach again then he's well enough to deal with his student's problems really, so you might well have an informal chat about the problems with him.
Some teachers who've not experienced a brace for themselves really don't appreciate the problems it can cause. I went through music college for one year with a new brace on my teeth (not recommended

) and made it work but it sure wasn't easy. However, it did help me appreciate better what my pupils go through now.
Hope you get this sorted.