I don't think removing grains would be of much use to him . Other people may have different ideas of course. But what obviously might help him is a "proper food" and good fibre diet, as any straining to defecate may make the hernia worse.
Grains like brown rice, can't do him much harm so long as his diet is basically meat/fish based ....even small amounts of wholewheat pasta (dogs love pasta, but of course not to much!) A few carbs and grains are quite healthy for them.
Then there are other grains like millet, buckwheat, quinoa (which is literally a seed, and contains protein in quite good amounts)
Barley is OK too.
Though Corn (Maize) has a bad rap for adding to dog food, and indeed can upset some dogs if given in whole niblets!...it is fine added if mashed thoroughly, as they can't digest whole niblets.
But grains shouldn't usually be more than about 15-20% of the meal, maybe less.
You sound like you have worked out a good diet for him.
You may want to research about supplements (vitamin/mineral) for dogs as maybe if he's having a home cooked diet, he needs some canine-specific extra nutrients? And Salmon oil in recommended quantities added to food is usually good for them.
Veg. like pumpkin, carrot and other veggies provide fibre too, and vitamins and dogs love veggies.
I fed my own dog a mostly home cooked diet. She did have a little healthy-ingredient wholistic kibble as well on the side, as she did like the "crunch" of it.
She did very well on her diet for many years.
Of course, she didn't have a hernia as your dog has, but the food she ate kept her nice and regular and fit.
I am also surprise dthe vet did not notice the hernia! If he did a finger palpation (presumably to check your dog's prostate gland? Strange....
But maybe the hernia has only recently appeared.....I don't know.
I can understand your reluctance to "put him under" again for another procedure. But if the operation is not too challenging, and your dog is very fit and well generally, and has good organ function and good blood-work, then the risks are small.
If by chance at the back of your mind you ever feel surgery may be necessary, then leaving a good gap between his neuter surgery and possible hernia surgery should be fine for him.