You could send him to school and see how he goes? If you find him struggling, maybe give him two years in prep?

FWIW - My brother was in the same boat and mum did send him to school. He struggled right through to yr 12. A teacher told mum when B was in year 8, that he would have been greatly benefited by staying back for that extra time. But I don't know where he was at developmentally before mum sent him to school. Good luck