It's good that he is able to talk about it, but it's not enough. He needs to figure out why this keeps happening. It is unusual for a young and healthy male, in a happy and otherwise fulfilling relationship, to lose interest in sex, and if you are certain that he doesn't have any issues with his health or his sexual function, the issue may be psychological. It could be something deep, such as a fear of attachment, and withdrawing from sex is a form of self- protection. Is it possible, consciously or subconsciously, that he is afraid that if he stays with the same woman for too long he will find himself married with kids?
It might, however, be something much more shallow. Sex and commitment have a paradoxical relationship. Sexual desire is, typically, predicated on novelty, so at the beginning of a relationship couples will go to any lengths to be able to have sex with each other. Habituation and familiarity can make couples lazy, and the steepest decline occurs in the first 12 months of a relationship. It has nothing to do with age - 60-year-olds who have been together six months are often having more sex than 30-year-olds who've been together for six years.
Sex in a long-term relationship is less spontaneous, but couples make an effort to do it because they know it enriches their relationship. When the hand reaches across the sheet in the early morning, they might fleetingly think, "I just want to sleep," but their partner's touch triggers the release of the feelgood neurochemical dopamine, and so they respond. Once you lean in to sexual desire there is no going back, and when orgasm pumps you full of endorphins and oxytocin, it makes you feel amazing - and strengthens your emotional connection.
Your boyfriend may feel that he can't force himself to "want" sex, but sexual desire is both a drive and a motivation. The drive to have sex is biological and he has little control over it, but the motivation to have sex is something that he has complete agency over. If he has no problems with sexual function, having sex to make you happy should be enough. If he can't or won't do that, it doesn't bode well for the relationship. However nice he is, why would you settle for a sexless relationship in your 30s?
He needs to work out why he is unable to sustain a sexual relationship, and then he needs to do something about it. A sex therapist or psychotherapist would be able to help him unpack what is going on. If he is not willing to address the problem, you may need to reimagine your "serious future", but remember that many couples go through periods of change in their sexual relationship. With mutual understanding and perhaps some outside help you can navigate this challenge together.