tenner at the most IMO and you wont get a lego set for that. the suggestion about asking your own son is sound - he's the same age and sex and might know what he would want. In my experience, party gifts at that age are just a token and could well get lost between the party venue and home, not opened immediately, or confused as to what comes from whom.
Fake tattoos, glitter tattoos or nail stuff, pens and paper, little games and so on are all good IMO