No judgement op.
My concern would be that your ds would be placed with Foster parents, who may not have the knowledge and experience in dealing with young children with autism, non verbal and has challenging behaviour.
If your ds is five in age it is highly unlikely he would be placed in a specialist boarding school placement, as he is too young.
What if the Foster parents cannot cope? The Foster placement will disrupt and what then? Your son is moved around from Foster carer to Foster carer becoming more confused and traumatised.
What have you tried so far when his behaviour is challenging? Is he like this at school? Can you identify what triggers his challenging behaviour?
My ds is autistic, verbal, but non functioning language has other complex needs and can have challenging behaviour. I understand how relentless and exhausting it is.
It took a long time, but you do have to parent differently. Therapeutic parenting works for my son. However, be prepared for lots of negative comments from people, who do not understand it.