Need advice on schools for autistic 13 y.o

Hi there, rather unusual one as my son has grown up all his life in Thailand and after many aborted efforts in that country was finally diagnosed in Australia with autism level 2 and adhd. My problem is the international school he is at - about to enter year 9 - have no special needs nor do any others that are affordable. On the plus side he is happy there and wants to stay. I’ve just found out there are no spaces to transfer him in to my Uk Lea. Given he gets incredibly low marks in all tests, understandable given his unique learning needs, would I be negligent to let him stay at his school until 16 denying him bespoke learning support and probably feeling a failure academically, or is the emotional side more paramount, being somewhere he is happy? Would love some opinions as struggling to make a decision that’s best for him.