Partner won’t stay hard...

So the guy I’m sleeping with can’t stay hard anymore... to give some context we are very close friends and coworkers of about three years and we were promoted to management together, and have a few on and off romantic/sexual encounters throughout the time, so we are pretty comfortable with each other. When we first started 2 months ago (this is the first time we have continuously and regularly slept together) it was awesome but it has progressively become where he finishes pretty early no matter how hard he tries, within like 10, if that, and then when we try again he’ll get rock hard, start having sex with me, and three minutes later it becomes completely flaccid literally during the act.

I know he’s frustrated by it, and he’s said this has never happened to him before(finishing fast and not staying hard).... I don’t want to say it’s me and I’m sure it’s not, but it’s one of those things where you go like, well what else can it be??

Anyone have a tip?