If he likes mashed potatoes, that is a start! I mix all sorts of things with mashed potatoes like sauteed & mushed zucchini (courgette), or shredded cheddar, or boiled and mashed rutabega (swede), or boiled and mashed peas (a favorite!)... all sorts of things!
Maybe he just wants to practice on some things? I know Philip loved chewing on celery. He never bit any of it off, but it was a good practice for him getting used to eating - especially by himself. Philip has had teeth from very early on so we've been careful about waht we give him since he's been able to bite off chunks since about 5 months old. :/ Have you tried bananas? If not little slices, maybe just mashed up on their own in a bowl? Try sprinkling some weetabix or wheat germ in it to make it thicker and crunchier. How about applesauce? We make ours homemade and put apple pie spice in it, Philip adores it. For finger food he loves little chopped up kiwi fruit, apple, and strawberries. We chopped them up into tiny little cubes to begin with and now they are bigger, as he has the hang of it (and plus he's 11 months old). Try sprinkling some shredded cheese in front of him and let him pick it up and learn to eat it. Something he can do on his own and have fun doing.