Ezz, you can clean them with either a wet face washer, or a baby toothbrush. Kids should not use toothpaste until they can well and truly spit out (ie not for a long time yet!).

Just make it a fun thing, and maybe let him have a go when you're brushing your teeth, so he copies you. Remember though that parents should brush their kids teeth at least once a day until they are about 6 or 7.