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
I was sent to school at 4.5 yo & IMO it was too young - socially immature and struggled throughout school. Sounds like your son however would cope well? Don't worry about him not being able to write his name yet - my sister is a primary school teacher and some can, some can't but she says that is what they are there for