If he is only 21 and only had one girlfriend it’s not that unusual. My nephew did not have any girlfriends and didn’t seriously date until 24 and he is now happlilly married. Your friend might have commitment problem or might just figuring things out and learning as many young people do. You seem to be hyper focused on what he does or doesnt do. Did he ask for help? He already said he thinks your interference looks like jealousy. He clearly doesn’t want interference. If he didn’t ask for help, then attempting helping with something that person doesn’t want help with is intrusive. Just be his friend and be there