Have you talked to him about it? I had a really bad sexual experience at an early age, and sometimes my dick just stays soft until I'm extremely comfortable with the person. I usually just get it out of the way early. "Hey just a heads up, sometimes I need to be really comfortable with a person to get hard. It has nothing to do with you. In the meantime you can use my throat all you'd like" followed my a sly grin. I just treat it like it's no big deal, because it really isnt. Sometimes I get shot down and it gets weird, but usually people are understanding and a lot of people like that the pressure is off on their end and we end up having a lot of fun.

Sex is supposed to be fun. Have fun however you can and things will come around.