Seems like he’s just really awkward honestly. Maybe I’m forgetting how he was introduced which makes him seem worse but some people are just really bad at stuff like this.If he really wanted to be friends, then why did he try to insert himself into their outing so forcibly? it only creates a bad impression that reeks of interior motives when someone you don't know well wants to hang out with you while you're with your childhood friend and little sister.
Just invite him normally in the classroom and make it a one on one at an arcade or something.