Hello and welcome to the forum. It is not unusual for dogs to become bored of dry food so you are certainly not alone with this. I think that the answer might be to continue with your usual food if your dog does well on it but to give him a topper. Obviously you would need to reduce the dry food to compensate for this. Chappie does not score well on the Dog Food Directory. You could do one of two things - look for a good quality wet dog food or provide your own suitable, home cooked topper. Tinned fish such as sardines is usually welcomed. There is a section on the forum about home cooked food that might be of interest. I top up with home cooked chicken, white fish, sardines and scrambled egg with plain yoghurt so it is not very time consuming but gives the dogs good protein. Remember that dogs like food that smells good to them so a little bit of warm water poured over the kibble immediately prior to serving will bring out the aroma and might tempt him.
By using the Dog Food Directory you can easily source a wet food that would fit in nicely with the dry food that you are already giving. Using these filters: Wet food, Rating 3.5 to 5.0, natural/clearly labeled, no red ingredients, returns four pages of wet food products so there is plenty of choice. Possibly the easiest option would be to use Wainwright's wet food as you already give the that variety of kibble.
Please ask if you need help with using the Directory or need to discuss particular products. As ever, we would be interested to hear what you decide to do and how things are going with your dog.