You don't have to send him to preschool at all. Why not wait and see whether he seems to need something you aren't able to provide, and look into it if so?
Because preschool is so widely used, some kids who don't go may start to get lonely. Mine did. As your son gets older, you will probably find that there are fewer and fewer children his age at the activities he goes to and in the park during the daytime, because most of them are at nursery.
My child wanted to go to nursery at three in order to have better access to children to play with. This was the only reason I sent them. However, though they initially liked nursery, there were some things they disliked and eventually they decided they'd rather not go, so we stopped. You can just trust your child to know what they need.
Preschool is sometimes promoted as a necessary, gentler introduction to school. But the Reception year of school is designed as a gentle introduction to the school environment. So if a child really needs a year at nursery beforehand to get ready for that preparatory year at school, to me that suggests that school is quite a poor fit for that particular child's needs, and ideally he should start later rather than earlier.
If you don't need the free childcare which nursery and school provide to working parents, you could just send him whenever he seems ready.