First of all, it bothers me when people chalk up bad behavior to age. I'm 23 and I've known how to be respectful for YEARS. My husband is 24 and HE has known how to respectful for YEARS. It may be an inexperience thing, but not an age thing.
Personally, I get really upset when people judge me based on my age. I have been discounted as next to useless in various social situations because I'm "too young to be able to handle it" whether it be conversationally, physically, or emotionally. And quite honestly I think it's crap. I have more life experience than many 40 year olds that I meet and can often offer something of value even if it is just a different perspective of the main topic without adding any real content.
I agree with a lot of what GG said, except for the focus on age being a contributing factor, because, well, as I've said I don't think age really matters but experience most definitely does.
Now, this guy may be young AND inexperienced which which would make him act jumpy and probably inhibit his ability to plan ahead. Everyone gets nervous and can act completely different when they are in new situations that push their limits.
Is this the guy's first foray into anything poly? It's quite possible that he was subconsciously "staking his claim" because that's what you do in mono situations. Unlearning social norms (like trying to tell people to BACK OFF, HE'S MINE without actually saying it) is difficult. I'd expect the same behavior from an older person who had never experienced being with someone in multiple relationships, too. There's got to be a bit of a learning curve - talk to him about the signals he was sending out, or if you don't want to, have your partner do it. Help the guy grow and learn from this experience instead of assuming he can do it on his own.