OR, since no one else will say it, i've been in a relationship before and we got it on like no other. Sure as shit, as I lost interest in her and realized that she wasn't what I was exactly looking for, the sex came to a damn near standstill. I gave the exact same excuses as your guy.
Now, i'm not saying that is what is going on here, BUT, you do have to consider everything that "might" be going on in his head. It took me months before I had a "talk" with her
Whiskey