My boyfriend and i are having problems in bed?

My boyfriend and i who are both 19 recently decided that we were ready to have sex. Now, i have had sex before i was in this relationship but he was still a virgin. The first time was great and we both really enjoyed it. The next time we went to have sex it was still fine, but we went to go another round a little while afterwards and he couldnt really keep it up. We both thought it was because we didnt wait long enough inbetween. The third time was sort of the same thing..except he couldnt keep it up long enough to even get himself inside me. Then last night we were having sex again and the first time was fine, but then an hour or so later we went to go again and it took him a while to get it up and then he came within 45 seconds. I dont understand this because any other time when its not intercourse he has no problem staying hard and he lasts a while. He