I'm sorry. Sometimes youngsters just don't thrive and there's nothing we can do about it. Personally I wouldn't get another animal from the shop - from the looks of the one you did get, they didn't do a very good job with him. There are a lot of reputable breeders in your area who will be happy to sell you a healthy youngster, and probably for less than you paid for the one that passed away.
In case he was sick I would thoroughly clean and sanitize everything in the tank before bringing another snake into the house, especially if you don't have a necropsy done.