Good job on the scoring. I like to score my own as well. Far as the age you'd have to measure the 1st molar from the gum line to the lingual crests in millimeters to be sure of his age. That would be the 4th tooth in line. The first 3 are premolars. If he's 3.5 the measurement would be 6-7mm. If he's 4.5 it'd be 5-6mm.
When I was in college I had to do volunteer work with the DEC at deer check stations. So I got to age several deer. I also kept my aging charts for my own personal use.