Good to hear about Marshall. And yes I realize diamonds are "cold weather" pythons but cold weather is a relative term. They still need a 90F hot spot at all times to regulate their temps. They are still ectothermic and especially after eating, they need to digest the food. now yes you do need a cooler spot than for say a boa or BP but regardless, they still need a nice warm spot. Of course your biggest issue is the hydration. Another thing is to maybe looking into a radiant heat panel. They don't dry the air like a bulb will. Anyways good luck and like mentioned, a vet visit might be a good idea too if he doesn't hydrate up.
I actually have not seen a Caresheets suggesting a hot spot of 90 for diamonds, But 85 is popular. Diamond pythons in the wild do experience a temperature drop, and in order to maintain their body heat they curl up in a tight coil.