I wouldn't switch to crappy food.
I would feed regular food or at least food that has lower fat, but not single digits low... but cut back on quantity.
You can cut back to 2 cups of food a day. If needed, you could even cut back to 1.5 cups (but absolutely with filler). It does not sound like much for a dog, but if you need to get that dog healthy, it is basically what you have to do. You can add green beans for the filler.
Take him swimming a couple times a week. And make him swim full out. If he doesn't swim full out (as in going well out into a pond or lake and working his legs), then you may want to check around for places that offer hydrotherapy. Some dogs out there wade around the edge of the water, and that's not enough exercise.
And do at least 1/4 a mile walks every day. Regular forced exercise is going to matter a big deal. Running around loose is not good enough because a dog who has lower energy is not going to burn too many calories by running around enough. If you can push that distance up to 1-2 miles, that's even better.
I don't do this, but conformation people build up their dogs muscles and keep them in shape by using water treadmills. And they will do this on a routine basis. <- That's probably getting kinda extreme. LOL. But it does give you something else to consider to keep your dog at a healthy weight.
If he's like my Jacks who goes through the same stuff (and is intact, so this weight gain stuff isn't completely to being fixed), just cutting a little on the meals, no treats between meals, upping exercise... helps get them to drop the pounds.
Both my guys had minor muscle injuries after a pretty long hike last summer and missed out on a lot of swimming that we'd normally do (I typically take my guys swimming 3 times a week and that used to keep both svelte). Bertie is young and did not gain too much, but Jacks did. He actually went up to 92# at the highest. Just exercise and diet alone got him to get back down to 84#. My preference is 80# with him just because he's at that age when tumors start popping out all over and tumors favor the fatties or something.