Hi N..
Though many mothers might disagree with me, I think you should rock your baby to sleep or pat him on the butt (while in bed, causing movement) till he falls asleep. I know it's easier for us "Mothers" to have them fall asleep on their own but, if he feels the need to feel movement to fall asleep and it's a matter of only 5 - 10 minutes - then why not help him get there? This will ensure he will go to sleep without having to bang his head around.
My babies (2 1/2 yrs. & 10 months) never did this and I hadn't heard of this before. It could be something completely normal but, I too would be very concerned. I know most babies enjoy movement - this is why they fall asleep in the car, the swing and while rocking them. My son will fight his sleep a bit but, once I lay him down on his belly and start patting him he begins to "sing" and falls right to sleep.
I run a very busy schedule, I work full time, have two little babies and a husband that thinks I'm a housewife when it comes to cooking & cleaning (wish he would think that when it's time to pay the bills) so I know it's hard to take those 5 -10 minutes to put him to sleep BUT, I do enjoy the bonding time. They grow so fast so I don't feel bad having to take that time to help him get to sleep, I feel tired but, I think it helps him feel more secure and closer to Mommy.
Again, a lot of Moms will disagree and say they should fall asleep on their own - I don't mind helping them while they're small. My daughter is 2 1/2 now and she's been falling asleep on her own since she was about 14 or 15 months with no problems. I just think they need a little help when they're smaller and I enjoy the bonding time.
Hope this helps.
Good luck!!!