Welcome NicosMom! Wow, it sounds like you have a very responsive and flexible school. Your kiddo got what a lot of us have only hoped for, or else we've had to do a ton of advocating to do it. It's nice to hear such good news in a first post.
As for the combined 2/3 class, it sounds like it will be perfect for your kiddo for next year. Based on what you've said with how quickly your son adjusted in the transition, it's possible that you may have a kiddo who will continue to progress quickly. It's a little too early to predict what will happen next year, but I understand your concern. I think the combo class will be good, Your child we be placed with others of his ability, which could be 3rd graders, if I'm understanding the set up correctly. So if he progresses well in the 3rd grade materials, the teacher/school may recommend a second acceleration (or it may be something you want to consider then).
Another issue you may encounter is with pace, a common problem for some GT kids who pick things up so quickly. If the school is not accelerated or a GT program, things may get a little slow for your kiddo. However, there are positive signs based on how the school has responded so far, and maybe you could ask them about curriculum compacting if pacing turns out to be a problem. There may be kids he can be grouped with as well.
You could also do a little research on any GT programs in the area, just in case. We ended up moving our DS to a school for HG kids to avoid a second skip in our local school, and this turned out well since the program is accelerated (one year advanced plus faster paced.
These are just my random thoughts. I do think it's early to really figure things out, but you can think about possible options. I would also recommend getting in touch with the school regularly about any and all concerns. They sound really wonderful, and I'm sure they will come up with some great ideas too.
