Hi,
I bought my 4 kids to the Canary Islands 2 years ago from the UK, the other posters are right you must not worry, Children are more adaptable than you give them credit for, the Irony is that the Younger they are, as yours is, the faster they learn, all my kids aged 4,6,10 and 15 now all speak fluent Spanish.

Put your Kid into school he will love it and make Friends instantly. the younger they are the less hang ups they have. Young Kids don't think in terms of Language they just "communicate"!

Regards
Adam