I know that for me, personally, when I was 13, I collected baseball cards and put together model cars. If he likes cars maybe he might like putting together models. I still build model cars and I'm 26. My son took an interest in them when he noticed that I liked doing them (he is 10, not much younger than your son), and now him and I work on them together. It is good bonding time.

If he isn't into cars, maybe he would like to learn (if he already doesn't know how) how to skateboard or rollerblade. Or if he is technically inclined, stores like Radio Shack have electronics project kits that they sell.

Keep me posted on the hobby situation with your son.