The constipated boy should not eat any dry food. That is what causes kidney issues and constipation.
He can have one to three ounces a day of 2% milk.
Canned food is much lower in calories than dry. It will manage his weight in a healthy fashion
No cat, especially an overweight one should be dieted. Weight loss must be gradual to protect his liver.
Although overweight can cause diabetes and other health problems, it is very dangerous for a cat to suddenly lose weight. That is because a cats liver becomes accustomed to a certain caloric intake. If that amount is cut (for any reason) the liver will think it is starving and grab protein and fat from the cats muscles and organs. This can cause liver and organ failure, muscle wasting and lead to a weakening of the immunity and a disorder called Hepatic lipidosis:
Essentially what happens is the body goes into a state of starvation, and a signal is sent out that the body must mobilize the fat stores to provide energy. Unfortunately, there doesn't seem to be a signal about how much fat to mobilize and fat cats have a lot... so it all gets sent to the liver to be converted from fat into glucose. And the liver gets overwhelmed and shuts down. This leads to nausea and vomiting, which means the cat won't eat, and the body tries to mobilize more fat. The cycle continues and the liver gets into more trouble.
So you can see why dieting would be more dangerous than a bit of overweight.
Cats are meant to eat protein so it will not harm them unless they already have renal issues.