I spoke English at home and Welsh everywhere else form the age of 5 to 11. By the end of it I could speek welsh better than English and couln't tell the difference unless I concentrated on what I/the other pweron/the book was saying!
I'd leave it for school, trying a third language with no context is just going to confuse him, at the moment I guess he barely registers that mum speeks one and dad the other, if they both start speeking a third lot of jibberish he's going to be confused entirely!
Aged 5 it took me about a term to pick up almost fluent Welsh, so if it bothers you find a private school that will teach him in a 3rd langage at that point.
