My bf and I have the best sex but it’s wierd to me that while having sex he will get hard and then get soft but not completely and continue non the less

And we usually have sex for hours (2-7)he normally cums at least 8 to maybe 10x, sometimes maybe more i always loose count

And he’s really into me and is like obsessed with my vagina

So I don’t understand how he can’t just stay hard during the duration before he cum