Honestly that is not you. It’s not really him either. That is just something that will happen from time to time and if it happens a few times in a row you may think it’s a problem but usually it’s just a coincidence or something that has more to do with technique or sequence of events.
I notice that might happen to me if I have had sex a lot that day (came like 30 minutes prior) or if I spent a lot of time on foreplay, then have sex, go back to foreplay, the sex, then foreplay, then sex again. Sometimes I just won’t cum but that has been the case since I was younger too when I am constantly restarting sex or just came.
I know that this happens more as he gets older in general as well, but assuming he is in decent physical condition and can cum from masturbation you are probably fine. If it doesn’t bother him don’t let it bother you.
Girls tend to blame themselves for this a lot I have found and it is never the girl. Guys can cum quickly sometimes and it takes forever other times. Sometimes not at all.
For example, if it is a morning session for me, I tend to last way longer than I’d like to and sometimes that means no orgasm. I really don’t give a shit if I cum other than for her own happiness. To me – I want to make her cum. That’s what I want to see.