Is your squirrel a juvenile, or an adult? Are you giving any source (block or nutritional ball) that provides a supplement of minerals and other nutrients with these foods?
Once tree squirrels mature their need of both higher calories, and greens to balance grain and berry sources, decreases. Even so they should be offered specific green foods that contain higher levels of calcium, with lower levels of anti nutrients.
Some of the best sources of calcium include the immature leaves of: Chicory, Escarole, Lettuces: (Romaine lettuce, Frise', oak leaf lettuce).
Calcium rich goitrogenic greens low in calcium lowering anti-nutrients (Mizuna, Bok Choy, Pok Choy, Kale, arugula), feed baby immature leaves, and for cabbage, or cabbage like sources, blanch them for 4 minutes in boiling water, which disables most of these calcium lowering compounds, and reduces the gassiness, and bitterness some also.
Add this source to the leaves, it is a healthy seed oil that gives greens a nutty aroma and taste, that dampens the typical bitterness of their flavor.
https://www.amazon.com/Pumpkin-Seed-...mpany++pumpkin