I think when he said 31 years old, he meant to say “I’m mentally thirteen”. That’s such a disrespectful comment.
There’s likely two things at play here.
He may be on the smaller side and your bodies just may be built differently than what you’re both looking for. Not your fault.
Most likely, he’s given himself death grip from masturbating too frequently and too tightly. I’ve gone through the same thing, and cutting back on masturbation helps tremendously. The more penis havers masturbate, we tend to squeeze harder to chase the same sensation we have prior. Thus getting ourselves to the point where we’re trying to juice it for a smoothie almost.
Regardless of what though, if my partner told me she couldn’t cum because I was too small, I’d be fucking crushed too. I’m so sorry he was so unkind and I hope there can be amends made IF everything else in the relationship is healthy and you’re both happy and feel respected.