I was told that my father was not really my father when I was in year 4 soooo when I was 9. I was only told because mother had split up with him and my other 3 siblings (his biological kids) went with him on weekends. It was an awful situation and my mother never had conversations with me about it but I'm sure you will handle it a whole lot better than my family who have always had communication issues.

I would have liked to be told earlier esp because the split was not very pleasant and my sisters were absolutely horrible to me when they came back after visiting their father (you're not our real sister blah blah blah).

Anyways I say communicating in a language suitable for your son now and then keep communication open for when he wants to ask questions. I've had the most wonderful step father for the last 25 years so it all worked out for me. The only time mother has mentioned my biological father was in a letter when he died and I only found out his name last year.