Rather than clubs for little kids have you thought about Science Fairs for older kids? I assume he would probably enjoy looking at the different projects and the kids presenting might enjoy talking to someone who is genuinely interested in their topic - even if that someone is only 3.5. They might be likely to get the words he's using and actually talk to him. Heck, you may encounter some kids who were similar when they were little.
Is there a science center or museum near you? Even if he's not mature enough to participate independently in a group he may enjoy presentations, movies, lectures, etc on his topics of interest. The other people attending - child or adult - would by definition be interested in the same topics. I realize that this is different than finding friends with similar interests but at least it could help relieve his current frustration. If there is a university nearby you may also find groups there. Again - he won't find same age peers but at least he could talk about the things that interest him with people who will understand. Maybe attend a rocket launch on a weekend?
Just a few ideas. Hopefully he can enjoy be the younger kid that an older one is "teaching" for the time being. Good luck!
