Speaking from past experiences, the anti-nail biting polishes and cream don't work particularly well. They could solve the problem - but in most cases they won't. That said, there is no harm in trying it.
I was a SEVERE nail-biter my whole life, up until about 3 months ago. On a whim I tried a hypnosis program - and after that the urge went down about 90%. Hypnotherapy, in my opinion, is far and away the best solution. Would it work for a 7-year old? I believe so, in fact he may enjoy it. FYI many children do outgrow their nail-biting tendencies, but a fair portion won't (and I certainly didn't).
This is the Hypnosis program that I used. Easily the best $10 I ever spent:
http://www.amazon.com/Stop-Nail-Biting-With-Hypnosis/dp/B003VYBJUC/
I've also found keeping a rubber band on my wrist & snapping it every time I have the urge to bit serves as a nice reminder. Hypnosis in conjunction with a rubber band & non-bite cream or polish is almost fool-proof.