He is definitely NOT a year old ball python at 100g's, unless he has been severly malnurished, but still I would think mouse pinkies would put more weight on than that.
Pet store employees are notorious for BS'ing so to speak, I know because I used to have to do it. Stores don't train employees in all the husbandry aspects of every animal they carry.
Your temps seem good, but how are you measuring? What kind of thermometer?
Your enclosure is too big, but you can make it work. Add extra hides and enclosure furniture and even crumble up some newspaper and put it through the cage. This will size it down for him and help him feel secure.
What are you feeding him with or trying to? The first concern is to get the snake eating, before worrying about convience concerns of F/t. I would size down his enclosure doing the above, or you could even got get a seperate 15 qt tub and do it that way. Give him a week then try a live hopper.
Monitor his weight.