I agree with Sabrina.
You say you have left some details out, which is fine, but as we know "the devil lies in the details".
Why would you question if he really misses you or just misses the sex?
I would like to think that he does really miss YOU though. Going from seeing someone 3 times a week to once a week is not nice, I should know.
My only advice is don't jump to conclusions, rather be bold and ask him if he misses you or the sex, but in a way that doesn't suggest that he misses the sex. He might get defensive if the latter is not the case. Goodluck!
PS: I'm a guy