Sex problems

This is also a lil TMI haha.

So I recently got an IUD and my boyfriend and I started having sex. The first time was amazing but it came outta nowhere (just cuddling and then boom) and he was like mindblown how good it was. But then the next day we tried again for like half an hour and he couldn’t do it at all. Like, he stayed hard but he kept switching positions bc it didn’t feel as good for him and the position from the day before wasn’t working and he got kinda frustrated.

I enjoyed it but he stopped so much because he wasn’t and now Im just kinda bummed. He’s fairly new to sex, mostly because he can’t stay hard w condoms so we had to wait for an IUD.

Anyway, any ideas what’s going on w him? How can it be amazing once and then not work at all the next? Help would be v appreciated :)