It might take time with his age, but I also recommend switching him to a pellet as the main diet. Harrison's High Potency Course would be the best one for a Grey, but you often have to order it online. You can buy Zupreem in pet stores, but Greys do better with the higher calcium in the Harrison's HP. Merlin, my Grey, also likes a bit of Golden Feast thrown in, which you can buy at My Safe Bird Store, along with the pellets.
It might take some time to get him converted since he is used to the seed, but I did get my 29 year old amazon on pellets. First just put a few in his food dish so he gets used to seeing them there. Then over a period of about 2 weeks slowly add more pellets and less seed until he is completely eating the pellets. If you see him eating them readily sooner than that, just stop the seed then.
And yes, veggies, fruit, pasta (they really like tomato sauce, but no hamburger), rice, whole grain bread, cooked chicken or turkey or chicken bones, cooked beans. But, if you make 70-80% of the diet pellets, he will get the nutrition he needs from those and the other foods become more just for fun and something to look forward to.
Also other nuts are better than peanuts. Almonds, walnuts, brazilnuts, hazelnuts and pecans have better nutrition and healthier fat. But, go easy on all nuts as they are all fattening.