I don't see you as a problem for your son's double faults. My son is 16 and he did not play his 1st tournament until he was 10.5 yr old, just like your son. Even though he is playing 18s national events now, he is still working on his serves, because double fault is often the main cause for a tough loss. Here are a few tips that I hope would be helpful.
- Serve practice. We use a bucket that holds about 80 balls. After warming up for 1st and 2nd serves, I asked him to serve 1st serve, followed with 2nd serve. If he misses 2nd serve, he would serve 2 more 2nd serves - alternating serving to the deuce side and to the add side. Do these for 60 balls. For remaining 20 balls may be used as his opportunity for a reward. You offer him a dollar if he keeps his double faults to under 3 (or 2 or 1 or 0). If his 1st serve is in, continue with 2nd serve...
- Serve technique. Don't assume that his coach is teaching him how to serve his 2nd serves correctly. For 2nd serves, the focus is the spin - one needs to hear that beautiful string pops. Search YouTube video of top pros' serves (1st and 2nd) in slow motion. In privates, ask his coach to focus more on 2nd serves.
- Serve Strategy. At 12, he needs to focus on spin and placement. You may ask him to serve only 2nd serves in a few matches so that he would have 2 chances to get a 2nd serve in. Very few 12 yr olders are good at attacking 2nd serves. Also ask him to take a few seconds, like bouncing the balls 7-8 times, before serve his 2nd serve. If he has a long rally in his previous point, he wants to get to his towel to catch a little breath before serve.
- Mental Toughness. If he loses a match because he serves 20 double faults, you may want to focus your post-match discussion on other things, what he did well and what he did not do so well. Don't remind him about the double faults because he already knew it. BTW, continue to work on other aspects of the game. If he can break his opponent's serves easily, he would be less stressful when he is serving. The outcome? He would serve better with less double faults.