Check prices on line at dealers in CO and CA. If it’s a new car, there might not be much difference. If it’s a used car, there can be significant price differences due to regional preferences. Again, check on line to see what the dealer’s asking prices are. If you decide to buy in CO, contact the CO DMV and explain that your son will buy there and move to CA. Your goal is to avoid paying taxes twice. See if they can explain how to do that.