That simply isn't how this works because his uncertainty is dropping, although maybe too slowly in your eyes. Yes, he will always have an inflated rating if he only plays with you guys. The only way to fix it is to have him play without you guys. It wouldn't take that many global 1v1s to fix this issue as long as they're reasonable. Meaning don't have a 2k global player who doesn't want to be 2k play him and let him win or some bs like that. This is not an issue with the rating system. The rating system simply isn't built to handle this situation, nor should it be.
FAF has been around since 2014. There are plenty of people who only play relatively low level premade team games. There's not a single instance in the past 8 years of some random dude hitting 3k because of this or whatever.