Had the 22 year old come across as mature (and without digs, jabs, a competitive nature and demonstrated a jealousy streak in our first meeting about my and my partners relationship there wouldn't be an issue, and I would've considered him a full-blown adult, not just legally.
Being 22, he's gonna just BE all Luke Skywalker. Act first, think later, go, go go! Very passionate about certain things, not fully gripping nuances of every social interaction, totally not a Jedi Master, etc.
Since you two are in the 40's and he's in the early 20's, can't we chalk this up to there being age differences? His posturing and whatnot?
I'd let it go and give him the free pass of first meeting heebie jeebies of his own.
If this comes up again, tell him to chill a bit. Assure him you aren't blocking him from BF or anything, you are happy shared BF is happy, and wish him well too as your meta.
If he does anything that is odd, just speak up to him to clarify. "When you said/did ___ I heard/felt ____. Was that how you meant it to be?" And invite him to do same with you. He's a meta -- I doubt your hinge BF expect you to be best friends just because of that alone if there's no connection besides that.
He doesn't have to be horrible and he doesn't have to rock your boat... but we do teach people how we want to be treated.
Being this young, I don't know how many polyships he's had -- so consider him with these glasses -- "Assume good intent. But perhaps sometimes clonky in execution." -- just as you expressed.
He won't be 22 forever. He'll outgrow being 22.