I think an important point that the OP and many of the commentators are missing is that masturbation, while prompted by sexual thoughts, is not always about sex at all for guys. Certainly sex doesn't replace masturbation for most guys, as they are very different experiences.
Sure, single guys, or those not getting enough sex, will masturbate for release of sexual tension, but guys masturbate for a miriad of other reasons that are not really related to sex at all.
It can be a powerful 'personal space' thing - even if you are chatting to a stranger remotely or using porn, masturbation is usually all about you - so almost diametrically opposed to the intimacy of actual sex. That can be a very comforting thing, and provide much needed 'time out' from the stresses of life.
Guys will sometimes masturbate if they are feeling nervous, or anxious. Since in this case it's a morning thing, could he be having a hard time at work? I know I've used it in the past to be able to prepare my head to face the day, in a way that sex would not provide (as sex intrinsically involves being ready to deal with another person psychologically)
It can also be a relaxation thing.
The fact that he is clearly happy with you sexually, and yet doesn't feel comfortable with you joining in, would suggest to me that it is this kind of masturbation (a need for personal space and time) rather than a sexual thing. It certainly wouldn't suggest to me that it has anything to do with his feelings or attraction to you.
However, obviously jerking off every morning could well be why his sexual appetite is not high enough to satisfy you, and the compulsive/secretive nature of it might suggest it is prompted by an anxiety or insecuity, or life pressures, that he doesn't directly identify with the experience.