Cat gaining weight while on a diet.

Okay so my Murray has been doing well since his bout with colitis back in March. Well, now I'm not so sure he is still doing well. He is a 1 year old neutered male. I have been taking him in monthly for weigh ins at the vet. His doctor told me to have him lose a pound when he was originally brought in. Yet, last month he was up to 15lbs, so he gained. The other day he was up to 16.18 lbs. His food has been cut back to 1/3cup 2x a day (he does not like wet food). He was eating Blue Buffalo freedom, but now his food is half Blue Buffalo and half Merrick Purrfect Bistro healthy weight formula (eventually he will be completely weaned off of Blue). Yet, I have noticed lately that he rarely plays. I can rarely even get him interested in his teasers and balls. I've bought toys so I could get him to exercise but he does not want to play. Albeit he was hyper and running around my room last night. Could his inactivity be due to his weight gain, or do you think both could be related to another problem. He still eats and drinks and poops just fine. His doctor is on vacation and I left a message today for another doctor he has seen before, yet no call back. What do you all think?

I was thinking maybe his new food and I should just put him back on the Blue? And as for being 1, I imagined he would still be hyper and want to play whenever a ball was thrown or a teaser was hanging and swaying right in front of him.