I went through this with a (now ex-)boyfriend, and here's my take on the situation.

It sounds like your boyfriend was the youngest of his group of friends,and I know when my ex, and then when I, turned 21, we were both the youngest in our respective groups. That meant that he had been left out when all of his friends went out to the bar, and now he can finally join them.

Plus, a lot of times, when you first turn 21 (say for the first 6 months or so, depending) the bars are just the COOLEST thing on earth.... trust me, it'll wear off after a while.

My advice, and what happened with my boyfriend and I, would be to explain to him that you feel left out, and since you only get to see him on weekends, you'd like him to maybe set aside one weekend night a week to spend with you. He should be willing to do at least that. And don't worry... like I said, the bar phase will pass pretty soon - it always does!