While he is at the minimum for safety standards to be in a
booster, I would still like to recommend keeping him in a harness. It she has headrests in her vehicle, you could look into an Apex 65.
Graco just released the
Graco Nautilus which will harness to 65lbs, then
booster to 100lbs. The top harness slots are supposed to be 1" shorter than the
Regent (18"). They are in most WalMart stores and run $129.99.
If you are set on getting him into a
booster, the
Britax Parkway is what I would highly recommend. Holds up to 60" and 100+lbs.