Perhaps he's trying to make friends with the wrong people. An interested person is engaged in conversation with the person they're with, not checking out every 2 seconds. Maybe he should try surrounding himself with more like-minded people.
Doesn't hurt to tell him. Unless he's the type that only wants someone to tell him what he wants to hear, in that case I wouldn't even bother.
If asked, always be honest. Sometimes we genuinely need to know so we can work on it. If you weren't asked for an opinion, don't give one. We aren't cool when someone is pointing out our shortcomings. It hurts our feelings