Soaking can be an indication of a number of things you should check into:
A) Snakes occasionally soak before shed to help loosen the skin. (Check to see if he's going into shed)
B) Snakes soak if they are too hot and need to cool down. (Double-check all the temperatures in your enclosure. What are you using to heat your tank, and how are you measuring temps?)
C) Snakes soak if they have mites on their skin. (Thoroughly check your snake, its enclosure, and the water for signs of mites!)

Glad to hear your new buddy ate f/t right away! Might be worth mentioning, but don't handle him until 24-48 hours after eating, and you might take it slow this first week while he gets settled into his new home.