Guessing you already don’t use condoms? You could try using lube which increases sensation, having him masturbate less often, either of you starting/finishing him off with hands before or after sex, stimulating his booty hole/balls during (the male ‘gspot’ is in the prostate), trying new things. It also could be a mindset thing, like if he’s not relaxing during it or is stressed in general that could prolong it. I wouldn’t keep going after he’s already orgasmed too or if you’ve had enough.
If you try all those things and it doesn’t help it may be a good idea to go to the doctor x