This thread seems to be relatively dead, which ain't too good. Probably isn't closed off...
There is a condition- I think the term is priapism- that results in this sort of thing. Apparently quite painful. It's also possible that it is sexual and he has hypersexuality, which doesn't result in constant erections but does make cause the sufferer to aroused more than can be considered healthy.
That, or any other medication he's taking has been switched with Viagra...