[Moppie]

You might be able to position the lap belt such that is well accross his hip so that he can not slip under it, then use the crotch belt to hold the lap belt down to stop it sliding up in a crash.

You might want to try and get hold of a motorsport association in your area, or a local car club with an active motorsport based membership, and Im sure you will find someone who can give you soem advice.
Its not the sort of thing you want to get wrong.