The only way I see that this isn't what it looks like is if (1) his mom threw him out or (2) the charge isn't for a hotel room, but for a meal or drinks.
In your place, I'd call the hotel and ask them to fax you a copy of the bill to rule out (2). Then I'd ask his mom if he stayed that night to rule out (1).
Assuming you confirm that he stayed in the hotel and lied about it, then I'd confront him: "Gabe, I know you've been lying to me. I don't know what you want from our R, but I know that I want an honest and exclusive R. That doesn't seem to be where you are. But I'm not willing to live with less."
