I wouldn't recommend diet foods in general as they are mostly just a bunch of fillers.

What you can try is replacing some of his portion of kibble with green beans or canned pumpkin (not the pie filling). So if you take away 1/4 cup of kibble, add a 1/4 cup of green beans etc.

Also don't go by the bag as to how much food you should be feeding your dog. Most dog food bags list portions on the high side.

Some other things you can do are to feed several (2 - 3) small meals a day, and gradually cut back on portion size, and weigh him every couple weeks to see how his new portion is working for him. You want the weight loss to be slow. Most vets will let you walk in and weigh your dog, no appointment necessary.

And of course there is always the option of upping his exercise.

Good luck!!