Neutering effects hormones so it can obviously change metabolism but in my experience very few change much.
Your dog is just coming up on an age where you should expect to see him fill out a bit. You might also need to adjust how much you are feeding as he is no longer a growing puppy. When I say he should fill out I mean muscle wise, you should notice his chest fill out a little over the next year. But he should not start to feel different/thick in his ribs and hips. If you have a hard time monitoring weight by the feeling then you can do it by actually weighing him.
A dog that size (30 to 45 lbs) shouldn't very his weight by more than a pound or two if you are being consistent with diet and exercise. If he does then you should cut back on food or increase exercise.