I am currently here on a single entry ED visa and my son is on a dependent visa. Our first extensions of stay will be at the first of April. Yes, I understand that he doesn't technically need one because he is under 15, but I'm not taking any chances and I'm getting him one anyway. Shortly thereafter (about a week or so later) we will be leaving for the US so that he can spend time with his dad for a couple of months.

I will be returning to Thailand in one week, and I've already cleared the trip with my school, so I'm not worried about me. I know that I just get a re-entry permit for each of us at immigration before we leave. However, he will be in the states for 2 - 3 months. So, my question is, is there a particular length of time that his re-entry permit is valid for? Or would it be easier just to get him a new dependent visa while we're in the states? Does it make a difference if he's still there when I do my next extension of stay?

A re-entry permit will only be valid for the length of his extension. If he is not back before the re-entry permit expires he will need to get a new non-o visa.