As your dog is now of a nice weight, it might be the right time to move on to a normal adult food. Because of the tendency to gain weight, I would probably go for something in the range of 26% - 30% protein and fat 10% to 14%. That should not upset his digestion as it is just a small step up from his current food. You can give higher protein/fat products but would need to watch the amount carefully as your dog would probably require smaller quantities.
Using the Dog Food Directory
I have used the following filters (they are on the left hand side of the screen):Type of food - dry complete
Food properties - natural and clearly labelled
Rating - move the slider to between 4 and 5 stars
Avoid ingredients - all red ingredients
Nutrient levels - move the slider to 15% - 30% for protein and fat 0% to 14%
On clicking 'Go' it produced a page of products:
Ancestral Canine Premium Senior Lite
Gelert Country Choice Grain Free Adult
* Lupo Natural Swiss chicken
* Markus Muhle Black Angus
Millie's Wolfheart Utility Mix
Nature's Way Mature and Light
Pero High Meat
Platinum Lamb and Rice
Wainwright's Adult, Small Breed and Mature Grain Free
Any of these would be a suitable choice. One or two are available in store, notably the Wainwright's which is Pets at Home own brand. Note that the ones marked with an asterisk are cold pressed so are slightly different to kibble but they are good quality products. We have a thread on cold pressed foods here.
It is important to weigh these products as they are heavier than kibble so the amount seems smaller.
You can adjust the filters yourself and in doing so you will get more, and slightly different results to the ones above.
Please let us know what you decide to do and of course, please ask if you need more help in choosing something.