Thanks guys.
He's definitely not injured or sick and I wasn't too concerned until he began losing weight. The only other garter in the house is the baby radix but he was like this before she arrived.
When I turn the lamp off for a while should I leave him at room temp (60-70 F) or put him in the basement which gets a bit cooler? Should he be moved from the other reptiles? He's on a shelf with my Knight Anole and some other reptiles so I'll still be out and about even if I don't interact with him.