relationship advice

I need advice, say that you just moved in with your boyfriend about 4 months ago, he seems like everything you want. & you think he's the one for you. Y'all go downtown, or out on the town. & as soon as he starts to get drunk a little, he starts doing dumb stuff, like trying to buy a girl a drink when you're not around, catch him. He denies it. & you let it go; but go out again... and did I meation this time was MY BIRTHDAY? I walked away to go to the bathroom, and when I come back he's dancing w/ a girl. What the heck am I suppose to do? Do you think it's more of the drinking or he's just not that into me anymore? But now, I can't stop thinking about the stuff he did, & I really wonder if our relationship is ever going to get back right, or if it's already ruined? Any advice?