Just sounds like a behavior you can work on. The Sheltie where I work does not like anyone to reach over his head to pet him, so I just tell people, and they reach to the side. But this dog you're thinking of adopting is still a pup. Sounds like a fear issue, which may disappear once he's secure with you. Some good obedience work and training, and I bet he'll be fine, especially as he just does it with strangers.