I've never had to vary the diet in older dogs - and over the years have had many, often several at one time - only change is they've tended to drink more which has sent me running for the vet 'in case'.
So I agree to get full bloods done, read the labels on all the cans and dry food you are giving.
The amount you are giving him seems slim. I am trying to visualise half a cup of kibble. Grain free is good and feeding twice a day is good too. I've always done it. For one thing it helps control their weight and as pets breaks up the day and gives them something to look forward to !
My current old people share five big handfuls of kibble and half a can of Pedigree Chum for breakfast between the two of them and then a lighter supper of the rest of the can of chum and 3 - 4 handfuls of grain-free. He gets the lion's portion. They get a Meradog biscuit (hard tack !) when they come in from their run in the woods and occasionally a vegetable or two if I've cooked too much. If your half cup sounds less than this, maybe you should increase quantities but ask the Vet and have bloods done.