That's great that he likes it. It's very hit and miss with chain bits. A lot of people don't go to them because a) They don't know how they actually work, b) they think it "looks harsh" c) Its outside the status quo to use one or d) their horses do not like them. Or at least, that's what I notice from being around a bit at the barn and talking at the races. I have no problem using one if that's what the horse wants. However I have only run into a couple horses at the barn who actually go well in one. Others find it to have a strange feeling and a strange action. My mare now doesn't like it; She will only tolerate a lifesaver mouthpiece.
Having the vet out couldn't hurt. But after you rule out that, then perhaps it's just because he is feeling fresh.