What support is there pre-diagnosis

I have a 5-year-old child who seems to have additional needs that differ from his peers. He has previously been referred to the neurodevelopmental pathway by the GP, however due to not having text book traits and being so young, the referalls all got rejected. Fast forward to now, he is having serious issues with school refusal; he doesn't talk in school either (possibly selective mutism, as he doesn't speak to anyone outside our immediate family), and he has a lot of anxiety. 

I have done parenting courses that the health visitor suggested as a form of early intervention, but they have now advised that there is no more support I can obtain. I am speaking with the school, and they have advised that there are a lot of indications that suggest that he could have ASD (like his older sibling); however, due to masking in school, they currently want to monitor him. This is fine; however, what should I do in the meantime? Every day, he complains he feels sick, gets upset and refuses to go to school every day. I still take him; however, I feel like this is very traumatic to all involved. His teachers say he's fine when he's there; however, due to not talking in the school environment, he wouldn't voice if he hasn't. I Just don't know what to do for the best. Any advice?