Here's a couple ideas. It might just be a stamina issue. If you spend a lot of time on oral, he might not have much left for intercourse. Have him kegel.

It might just be performance anxiety. He doesn't have to do much for oral, but might lose it a bit if he feels he has to perform.

It might also be that he just needs the extra friction that naturally comes with hands, lips, teeth, etc during a blowjob.

Of course, if he can't really get a hard on, it looks like he could get a job in most porn. They have floppy cocks everywhere. Maybe its normal.