My son turned 3, but he is above average for height and weight. He is at a healthy weight but most of it is his height. He is taller than most 4 year olds. He looks like he is 5. I don't know whether to put him in a booster seat already. Please help. I've gone online and asked but there isn't much help. Everytime I put him in his seat, he says oohh, mom my neck. Its at its highest. What should I do?