There’s prob a lot of nuance to this that you either have to talk with him about or give us the details on to go further..... Starting with has he always been like this..?

Tbh for my personally my libido can fluctuate from needed to cum 3-4 times a day to only 2-3 times a week, so many factors contribute to the change. If I’m depressed or stressed, if I have something in my life I’m super focused on that’s taking up my time, the state of the relationship with my partner. Ect ect ect ect. It could be so many things, but to just jump to “should I ask if I can fuck someone else” is pretty wild lol.... esp at your age, it ain’t like you been 10 years in a broken relationship.

Just communicate with him, and move on if you guys can’t figure it out.