My gut reaction is to cut him a break. Maybe it is a ploy on his part, but the danger of assuming that's the case rather than taking this seriously is so much worse--there is plenty of time for him to learn responsibility and consequences, but you don't get a do-over on how you handle his disclosure of abuse. It's one of those pivotal moments where you figure out so much about other people in your life, and the resentment that builds if it's handled poorly--well...
It could well be that the pressure of the end of the year prompted this disclosure; he could have been suffering about this inside for awhile. It could be that the task of finishing everything seems so monumental that he doesn't know what to do except avoid it. There are lots of different ways to look at how he's acting. People are so complicated.
Can you talk to the school? It may be a possibility to take some incompletes now and finish the work (on a strict schedule) over the summer, provided you can document the mental health issue. The grades would be replaced and he could just walk the stage next winter or spring. Individual counseling with someone experienced with CSA would also be a really good idea.
If it's at all possible, I'd try to separate his behavior now from the irresponsibility he's shown before. I just don't see the harm in dropping the discipline for a bit and treating this like a crisis, because really--it is one.