My son takes methlyphenidate for adhd, can he also take fish oil?

Yes. There is one : study that showed a modest improvement in ADHD symptoms in children for whom Omega 3 & Omega 6 were added to their stimulant regimen. For multimodal treatment including behavior therapy & environmental manipulations at home & school, including "Section 504" classroom modifications, see CHADD.org.

SqueezetheFishTight!: Anyone say he couldn't eat fish?Feed him the real thing, if you can afford it, like salmon (m-m-m-m, yummy!) twice a week. And maybe the caps once or twice a week. Just don't overdo it without the advice of his doctor. Please try and make sure whatever you buy isn't riddled with toxins like mercury--it's sometimes hard to find out--look at the labels and ask the grocer. Any doubt, pass it up.

Assuming no allergy: Check out http://www.drugs.com/interactions-check.php?drug_list=1606-0,1749-3873 which didn't list any blatant interaction between methylphenidate and fish oil. However, it's always best to chat with & inform his prescribing doctor of everything you're giving him, eg all dietary supplements as well as medications prescribed by other doctors.