Sandy is absolutely correct. Moving around a lot is not a mental disorder. It is frequently a choice made by the person who moves. Someone who grew up moving frequently might quite naturally feel that moving around a lot is normal, just as someone who grew up eating steak every Saturday would feel like it's "the thing to do" to eat steak every Saturday.
There's an old saying that goes, "there's no accounting for taste." And that's most likely the case here as well. For him, moving on a regular basis is normal.
Solid relationships take time and effort. If he has a desire to establish a relationship with people, then he will be able to do that. Some people are hard to get to know, and others take a long time before they open up. Again, there is nothing wrong with someone who makes friends easily any more than there is something wrong with someone who is harder to get to know.
One of my favorite sayings is this, and perhaps it is relevant here: "If everyone liked vanilla ice cream, we wouldn't need strawberry."