Disappointing my boyfriend

My boyfriend always wants me to bend over and do doggy style, but it really hurts considering he is very well endowed. He wants to do other positions as well but they either just really hurt or I don't feel comfortable with it. Also, I've only given him oral once and I hated it because I gagged so much, but I really want to please him. Any ideas on what I can do or how I can talk to him about it with him understanding? I've tried but he thinks I just don't want to please him.