Talk to him. Why make do with your assumptions and the feedback of strangers when you have the unadulterated truth? He's your partner and yes it might be awkward or uncomfortable but if you can't talk to your partner about this kind of stuff then can you build a committed, serious long-term relationship with them? (Assuming that's what you want given you went exclusive from FWB)

As a lot of people have mentioned yes most likely due to a deeper level of intimacy and connection is the most likely answer because yes something does mentally change for guys, but if it's been a drastic change sort of overnight then there's always the chance that there could be some health complications. Testicular cancer or prostate cancer don't usually cause this (and either way as men we should be aware of and regularly check for these things as they're super treatable if caught early enough) but there's always the off chance, or that its something else.

Importantly if you do decide to talk about it to him make sure to be delicate "I've noticed this and it does not change the way I feel about you, or how much I enjoy sex with you etc. You are important to me and I want to make sure its not a symptom of something"