I think it depends largely on the child. Height and weight play a huge part, but the personality of the child is just as important, IMO.
My 8yo rarely rides in a booster; she just cannot keep still or sit so that the seatbelt is properly positioned 100% of the time. She's more comfortable in a harness, and I am, too. I won't drive on the highway with her in a booster- I cannot drive 75mph and keep checking DD's position in the booster; it endangers everyone in the vehicle.
My 5.5yo is tiny (30lbs, not sure of height, but she's the shortest in her kindy class) and I will let her occasionally ride in the Monterey- only 3 miles to grandma's house- because she sits perfectly in it with no reminding. As a true spare, no way. But for an occasional 3 mile ride in a residential area? Sure, since it's sometimes our only option for grandma's van.
DS is one month older than your little boy and there is no way I'd put him in a booster. I've known a lot of 3 year olds and I've never met one who has the impulse control to sit correctly in a booster, 100% of the time. Even the biggest 3yo who fits wonderfully according to manufacturer's specs can be killed riding in a booster if he/she can't/won't sit correctly- that split second when an accident happens is when it counts, and I wouldn't trust a 3yo to be that perfect, not even occasionally.