When you shoot a bow you are using muscles that you don't normally use in daily life. When I recently bought my new 70 lb. Bowtech I could not pull it back even though my old bow was set at 80 pounds. Different cam, slightly different muscles.
So I cranked her down to the lower limit of the bow, 60 pounds, and started shooting. In about a month, and cranking her up a little each week, I was back up to the limit of 70 pounds because I was planning a moose hunt. Since then, and since the weather is colder, I recently cranked it back down to about 65 pounds.
There is no reason that 55-60 pounds won't kill a deer. Remember it is not the poundage that kills, but the ability to place a razor sharp broadhead in the vital zone of the animal. For deer if you find a poundage that you can shoot consistantly with, with the best accuracy, I would stay there.
Keep practicing and be consistant in how you shoot the bow every time. He'll get there.