Whoa...that is strange! His neck looks a little off...and I don't mean the movement. I mean his actual "neck". But the movement is very odd! I've never seen that before. Definitely take him to the vet.

As for the constipation...my BCI used to get that when she was younger. I cut her feeding schedule back to 10 days to give her a little more time to work it through her system. She's been fine ever since.