First thing I'd do is get your son to a bike shop. It's possible that his own bike is not comfortable/ exactly the right size for him, even if it looks it to you - ie the pedals, though reachable, may be a little stiff to turn and this puts your son off. Let him have a go on some others at the shop. You may find he immediately does a wheelie!
My oldest son had several friends who couldn't ride their own bikes for a while. When they tried other ones, or grew a tiny bit in leg length or strength, all was resolved.
I had to stop my son taking his bike to the park when we met his uncycling friends. They got upset watching him ride when they were still struggling. This put them off having a go. My son would end up taking turns riding both their bike and his, while they ran off to play on something else. Have you considered taking your son, minus his bike riding twin, to the park for some practice?
Alternatively, does your son have any other contraption with pedals? a car, buggy etc? Can he operate these OK? If so, perhaps stick with this for now and go back to the bike in a month or two.
All my son's friends (age 6 - 8 years) can now ride two wheel bikes effortlessly. It just took some longer than others, especially if they had no inclination to learn this skill. I couldn't ride a two-wheeler till I was 10 years old. Never wanted to, so never bothered to learn.