Cornsnake not eating/starting to lose weight

[Cmwells90]

I have a snow cornsnake, he's been off feed since oct. He's eaten twice since then but they were small meals (1 adult mouse both times), usually he eats a small rat every 7-10 days without issue. For some reason he's gone off feed. I've added decorations to make him feel secure, the warm side is 85-88 cool side is usually 70's. Humidity sits between 50%-60%. He's started to lose weight over the last month, so far it's only about 15 grams but I'm not sure when I'll need to start worring. If he doesn't start to eat, and I do need to worry, what should be my first course of action. I'm already planning a trip to the vet for all my snakes in the next month, so maybe they'll have an idea but I wanted to run it by you guys first to see if we can get the ball rolling.