Not so hard to believe.
If he ate say 1700kcals a day, as he said he was, and at a starting weight of 10st 7Ib, then it's not unreasonable to conclude that after 18 weeks of training and 2500kcals (+800kcals/day), he could put on 14Ibs in weight (at 0.75Ib per week). That would be 5,600kcals...