Well this can't be good...

So my boyfriend had a vasectomy years ago, he's been having pain in his groin the last few weeks. Now the one side, the vas defrens is really swollen and inflammed. Now working as a medical assistant I think it's epididimytis, but as of now he's refusing to get checked out, which is normal for him unless he's dying. I'm concerned partly because it could be serious, partly because the main cause of that is an underlying STI... but we've been together since March of last year, and I know I've been faithful and screened for all that... anyone else have ideas other than the obvious fact that he may have been unfaithful?