E.C.
sorry, but as much as I can empathize with a child really wanting to have the same kind of booster as his friend, at 4.5YO and 41 lbs., I dare say most certified Child Passenger Safety Techs would recommend that he be in at least a high back booster or even a 5 pt harness seat. The high back booster will provide some side impact protection and should also do a better job of placing the shoulder belt properly over the center of the shoulder than a backless would. Plenty of good high back boosters (Graco Turbo Booster, Britax Parkway SG, Sunshine Kids Monterey to name a few) will convert to a backless booster after the child outgrows the back, and IMO would be more appropriate choices than a backless booster for a child your son's age/size