C.C.
I agree with the other moms that as long as he still fits in his 5-pt harness, keep using it. But at some point they do get too tall, and a car seat that is too small will not protect your child adequately. For my daughter we moved her into a booster when she was almost 4 years old. Like your son, she was taller than she was wide - she had about the same proportions that your son does now.
Interestingly, I have this giant 2.5 year old as well (she is 35 pounds and built like a tank, poor girl), and although she weighs as much now as her sister did at 4, the pediatrician told me that it is not really about weight, but about age. Their muscles and bones become much stronger between 2 and 4, so of two children of the same weight/height, the older one will be able to handle a seat belt, while the younger will still need the harness. So... like everything else in life, it will vary from child to child. As long as you have him in a properly sized booster or harness, he will be okay.