it is 263 grams 1yr old
Just want to double check that this is correct, a one year old that weighs 263 grams?

owner told me to feed it medium size rat
Assuming the above information is not a typo, a medium rat, 85g to 175g is WAY to large for your snake. And maxing out at the low end of that range a hamster is still too large. You should be looking at the largest being rat pups in the 20g to 30g range maybe even one step down called weaned. Photos of the animal with something for scale would help.

Though as it has already been said changing out food items, specially right after a move is a bad idea.

The 90 to 80 change you describe should be the gradient in the enclosure not a fluctuation. This means that there should be a hot spot under the hide that is at 90 and stays at 90 controlled by a thermoSTAT. And an ambient temp in the rest of the enclosure of 80ish day and night.

All the of information Deborah lists would be really helpful in narrowing the problem down even further. That being said there may be no problem. I once bought an animal that didnt eat for 2 months then began eating regularly once a week and has not slowed down since. It will eat when it's hungry but at 263g it should be eating more regularly.