Here's what your tub should look like. It's a 6qt tub, one hide, newspaper(or paper towels), water bowl. Get an UTH or a piece of flexwatt(1 ft of either 4" or 11", wired), and get a quality digital thermostat(Johnson or Ranco is fine). You NEED a digital thermometer too that also measures humidity. He cannot stay in that big tank, or he'll never eat. Constant roaming = lots of stress.
Your snake really can't afford to lose much more weight. Your priority right now is getting him in a secure environment where he feels safe, getting his temperatures regulated, and getting him eating.