Ideally kids should be rfing until 4, if possible. In sweden rfing to at least 4 has been the norm for a long time and they have the lowest rate of traumatic injuries in car crashes with young children of all developed nations.
There are no rfing seats that convert to good boosters that I am aware of.
If you want to keep him rfing, there are several seats that will work for him until he is mature enough for a belt positioning booster. Then you can buy a $50 turbobooster and be done. With your vehicles, I'd look into the complete air, graco myride, or true fit. You can install them more upright than 45 degrees which will allow you more room in the galant.
If you do decide to go ffing, I'd do a nautilus or frontier and that will be the last seat he needs.