There are lots of possibilities here. From a male perspective I'm having a real think about what it might be that would ever cause me to hold off from penetrative sex. I love giving long, lingering oral sex to DW so I completely get that. But I'm also aware that she ultimately wants me inside her - so don't hold back on that (and wouldn't want to either, from my own perspective!)
So I can't help thinking that there is some fear, some anxiety going on.
For what it's worth, in my own experience, I began to be anxious about sex a couple of years ago when I began to experience occasional ED. Nothing too serious but at odd times, suddenly in the middle of sex, and especially when DW was on top. The anxiety I was feeling actually made the problem worse. I began to avoid that particular position for a while, which was frustrating for DW.
I then discovered Viagra (and more recently Cialis), which has resolved the issue completely. But what had been a mild physical issue - I guess maybe due to me being in midlife - had become a psychological issue, which was causing me at least a degree of sex avoidance.
Could it be ED? Possibly. He's only 30, but it's something which can affect men of all ages, and for different reasons, whether physical or to do with body confidence from knock- backs in the past. But it could equally be something else which has been suggested by other pps. But it is definitely something.
Whatever it is, OP, the only way to find out and help him resolve it (which might be quite straightforward) is to get him to open up about it. To say how important it is to you and that you want to offer help, support, and encouragement. Maybe write him a note if that's easier?