Keep in mind that it is more difficult socially for a student in elementary school to be enrolled part time than it is for a high school student because of being in one class versus many. This is, of course, assuming that his school does the traditional thing of having the class stay together for everything. If they don't, and work more like a high school, then part time enrollment could work well. Or if they mix the kids up for specials, that might work.
Our district has a "homeschool school" that my kids attended. It offers classes two afternoons a week and additional support for parents who want it (as in, they will hand you a program to use--I would *not* do this with an HG+ kid though). It was a great place for them and me to cultivate friendships.
As for not knowing what you're doing...that's normal. Your son is young, and you won't do much damage whatever you do. Pick a math program, get books at the library to read, and have him write about what he's learning about. Add more when you're ready. And also be ready to be overwhelmed by the pace and need for change, especially early on.
If you need specific support, the Well Trained Mind Community is a good place to find it. There are a lot of folks there with HG+ kiddos. Here's a link:
https://forums.welltrainedmind.com/
