I would stay away from blackberries and raspberries because they are very seedy. Cresteds like peaches, watermelons, mangoes, strawberries, figs, and bananas. Don't give any citrus fruits as cresteds can't handle the high amount of citric acid they contain.
You still need to get your crested on the CGD so he gets all of the nutrition he needs, but a fresh fruit treat is good for them. You can also mix the cgd in with the fresh fruit (and water if needed for consitency) as a treat or to help get him eating the CGD.