he cums to fast

so my partner and I have been together since highschool and have a child together. he's 23 I'm 21. everything is amazing except one thing.... the sex. he lasts MAYBE 30 seconds. I love sex, I could have sex 10 times a day if I had time. and I seriously think female blue balls are a thing. He's been to the doctor and she said that he's overly stimulated.... like what? so we try having sex still partially dressed. still 30 seconds... try in full darkness... you get the point. anyway, I'm at my wit's end. I'm super faithful so I couldn't find a different man to screw, he's not open to the idea of a open relationship. so irritated. 😖any ideas?