What medication is he taking?
I'd assume that if it just started when he began taking the medication, that it's a side effect of the meds. I haven't heard of that particular side effect very often before, but he's autistic, and I know that we (I'm autistic too) can have *weird* reactions to meds.
I'll write this up under the assumption that what he's taking is a neuroleptic (typical or atypical anti-psychotic, Risperdal is in this class), if this isn't the case, then some of this won't fit. My first instinct would be to direct you to the Autistic People Against Neuroleptic Abuse site. Unfortunately, it's screwy right now, and is saying that you have to have a password to even view it. Do read the article at http://www.autistics.org/library/meds/ no matter what he's taking.
Actually, also read "How Psych Meds Drove My Autistic Son Crazy" at http://www.salon.com/mwt/feature/2007/05/18/autism_misdiagnosis/ . That *does* mention laughing as a side effect.
It's not just anti-psychotics you have to be worried about -- I reacted badly to even tiny doses of SSRIs (though not with the same side effects you're listing), and I think this can be true for autistics on any psych med (and I've even heard similar said with regards to some of us taking things like Benadryl.) He won't necessarily react badly to all -- I do fine on Neurontin, and know other autistics who are also on it, I also know of some who do find SSRIs useful, and a small number find small doses of anti-psychotics helpful (but I stand by the remark in that article that children should not be taking anti-psychotic medications).
We don't tend to be violent for no reason whatsoever. We do tend to be hypersensitive to things that you and other non-autistics might not notice (our sensory systems tend to be much stronger than normal, plus we can't filter out sensory input -- you might be able to block out background noise, but he probably can't). Then there's also the fact that certain aspects of neurotypical nonverbal communication seem aggressive from an autistic perspective. I don't know how old your brother is, whether he spends much time away from the house (at school or somewhere else), but there's a chance that he might be getting harrassed/bullied if he is. Any or all of those could combine to make him violent. The solution to that isn't medicating your brother -- it's figuring out how to make his environment less hostile to him!