A few things, are you positive on the starting weight, has he been wormed, what is he dung like?
If you can weigh him at the same time of day, this way your weight is more consistent, if he has not been wormed worm him, he should be getting between 14 to 21 pound of feed a day, but if he only gained 5 lbs in a month, it is not a consumption problem it is something else. If you can see what he weighs in a week, to see if he is gaining at all.