One thing I can't see anyone has mentioned yet, is that a lot of places will see you for an informal assessment ahead of doing a more formal voice trial. You would be given a much better indication of your child's potential, and how (if at all) they would suggest you prepare him. It is your opportunity to ask more questions about how it all works, what the commitment is, how you fit your family around a chorister. Assuming it is a choirschool you are looking at, have a proper tour of the school; you should be able to arrange this separately to open days if necessary.
Personally I think it is fantastic when the interest comes from the child spontaneously like this. But you will need to do lots of finding out to work out whether it might be right for you. Year 3 is the right age for boys, typically they can start as a probationer in year 4 and then, with luck, get 4 years in the choir before voice change (sadly lots, like my son, have to stop singing before or during year 8).
For the right child it is a fantastic experience and I love that they finish their time at a stage when they can then knuckle down at senior school and pursue academic interests with music taking a back seat if need be. Not that my two aren't doing loads of music still! My youngest can't join the chapel choir at his senior school till his 3rd year and it already keen.
Please ignore the people saying, don't do it if you can't afford to remain in private schooling. A lot of top schools - especially boys' schools - have very good bursary arrangements, and ex-choristers are attractive to them. You are quite likely to be able to follow a good choirschool education with a good senior school paying only what you can realistically afford. And most means test with regards to your other children, not just one.
I've experienced one day and one boarding chorister, feel free to message me if you want.