Hi there --
First of all, stress is a general body state. Even though your boyfriend may not be thinking stressful throughts while having sexual time with you, that doesn't mean he's not still physically experiencing stress, or that he's not subconciously experiencing mental stress. Sadly, it can't be turned off that easily.
Secondly, if he's having a difficult time around penetration but not during oral sex, then it could be related to performance anxiety. For example, he could actually be worried about his performance during intercourse or his ability to satisfy or "impress" you. Sadly, that's a cultural pressure that a lot of men experience around intercourse. In reality, intercourse isn't the gate to sexual nirvana for most women; few of us orgasm during intercourse alone, and most of us need manual or oral sex to climax.
It might help if he knew that, and if the focus during intercourse between the two of you switched from orgasm to intimacy.
Also, he may have performance anxiety because having had an un-cooperative penis at one time during intercourse, he fears it will happen again. That's stressful, and the cycle can kick off from there. Again, taking the focus off intercourse for a while can help.
Hope this helps,