Casey's favourite "treats" are almond slivers, pumpkin seeds and Gerber baby rice crackers (they're just 100% whole grain rice with nothing else!) But she also loves getting a piece of raw fruits or veggies like melons, broccoli, strawberries, cauliflower or peppers (with the seeds). Her favourite foods that she goes crazy for and will actually bit my fingers to get it, lol, would be chicken, salmon and hard boiled eggs

Overall it's best to get her switched over from seeds to pellets. Have you tried the conversion at the top of the diet and health section?
In general there is a crazy amount of foods for them to try, so sometimes it's just a matter of figuring out what they like. But since he sounds like quite the little stubborn seed lover, lol, it might just take him a while to aquire new tastes, but with some time and patience hopefully he'll learn!

They love their grains, brown rice is great, there are also a bunch of other grains you can cook up... millet, quinoa, amaranth, oats (whole, not rolled or cut), hulless barley (not pearled), spelt or kamut, teff, brown rice, wild rice, buckwheat. I usually use 5-6 grains (along with quinoa/kamut pasta) and 5-6 veggies, make a big batch of food each month, blend it and freeze it in mini portions to thaw each night for dinner, it might be a good way to introduce some foods to him as it's all blended up and he can't pick out individual things but will give him a flavour of a variety of foods!

Some foods to try...
Veggies like pumpkin, carrots, acorn or butternut squash, sweet potatoes, peppers, kale, dandelion greens, mustard greens, collard greens, turnip greens, spinach or chard, broccoli, celery, cucumber, peas, zucchini, green beans, tomatoes, bokchoy, carrot tops, okra, kohlrabi, spaghetti squash, cauliflower, corn, radish, brussel sprouts, escarole, endive, corn, beet root. Fruits like papaya, mango, strawberries, blueberries, pomegranate, blackberries, raspberries, kiwi, oranges, cantaloupe or other melon, nectarine, cherries, apricot, grapefruit, banana, pears, apple, figs, pineapple, etc.
Hope that helps!