Sad turtle behavior is usually a result of a turtle being too cold, lacking UV light, or going into a semi-dormant state (brumation), which can naturally happen when the days are shorter and nighttime temps are lower in fall and winter.
Replacing your heat and UV bulbs can help perk him back up, and if you don't have a thermometer already, you may want to check that the tank temperature is proper.
If you think he's bored and needs enrichment, live food is a good way to go, or you can hide treats for him to forage for (make sure you take out anything he doesn't find promptly, this can get messy). Anecdotally, some turtles seem to enjoy a natural looking background image taped on the outside back of the tank to look at. I can't vouch for that making a difference to my turtle, but it can't hurt!