You could choose to purchase a Diono Rainier, which goes up to 120lbs maximum in High-back Booster mode, it doesn't have arm rests but it is narrow in the base. The harness has an 80lbs maximum weight limit, hopefully your son will lose some weight if the medical condition improves or some medications can be reduced, that is if you are "car seat religious" enough to continue use of the 5 point harness thru 4'6" tall. Otherwise there is the Roosevelt Special Needs car seat, those come in 2 sizes, just google "Roosevelt special needs car seat", or the Britax Traveller EL might go up to 85 lbs only in harness mode, it's a super-special needs seat with extra padding and harness accessories. There's also an older seat called a Britax Husky but that's retired now. Go to your local children's hospital for a referral to their occupational/pediatrics equipment engineer, and those specialists can help fit your child for the special equipment that they need. Remember, even if your child is way over the avg weight limit for their age/percentile, it's the height that matters most while riding in ANY vehicle with a standard adult-sized seatbelt.