As a vet, I deal with these issues on a daily basis. Different vets have different opinions, so take this for what it is, just an opinion.

I recommend that all male dogs be neutered before they get too old. Females actually will have a lower metabolism rate and therefore gain weight and act "lazy" after spaying, but neutering males does not lower their metabolism. It may make them more sedentary due to not chasing females, and therefore not burn as many calories, but it doesn't make them gain weight due to lower metabolism.

Many male dogs as they age will get some prostate trouble, and neutering reduces the risk of this by 99%. Of course, if he gets some problems, you can always neuter him at that age, but he might be old at that point and it could be an increased anesthetic risk. Neutering doesn't reduce the risk of cancer of the prostate, but that is pretty rare anyway. What it does reduce is the benign prostatic enlargement that is testosterone dependent. This problem is fairly common in unneutered males. It also pretty much eliminates some skin tumors (adenomas) that can develop around the rectum, also pretty common in unneutered males.

Neutering doesn't change the personality of a male dog. It may "mellow" him somewhat (make him less aggressive to other animals and people), but it doesn't change his basic personality or make him lazy.

I ususally recommend people who are trying to get their dog to loose weight replace high calorie treats with things like green beans and carrots. Lots of dogs like these and they dont' have many calories. Science diet also makes some good low calorie treats.