I had the problem with my wife for many years. Honestly it took years of patience, persistence and masturbation to get over the issue. Your level of tolerance may be different.
IMO when a guy lacks interest in sex there are two likely reasons.
1. Low testosterone
2. Performance anxiety
The low T problem get a lot of press these days and your partner seems too young - but he should get checked.
Since the likely issue is performance anxiety - you patience, support and communication can go a long way. This is also where integrating toys into sex can be helpful.
We made a lot of progress in our relationship when we stopped looking at sex as 'making love' and treat it more like recreation. In recreation who have toys - and we have a couple of drawers full of sex toys to keep things interesting.
I found that like with drugs - you need a 'gateway'. For drugs it's often thought to be marijuana - for sex toys it's your basic non-phallic clitoral vibrator. It helps you orgasm - and isn't a threat to his masculinity.
From there we climbed the ladder and have all manner of dildos, vibes, slappers, pillows, strapons, etc. Once you get started - try to move the dialog steadily forward to the areas you are most interested in - and help him develop new interests.