It is absolutely imperative that you get a thermostat. As Aaron said, this is NOT optional. A t-stat should be the first thing you purchase after the cage itself. They do not have to be expensive. The Hydrofarm is a decent one on Amazon and you can pick it up for around $30. http://www.amazon.com/Hydrofarm-MTPR...arm+thermostat
Even if your boa is thin, feeding him more is not always the best way to put weight on. Boas have much slower metabolisms than BPs and do not handle too-frequent feedings or large prey sizes well. Feed APPROPRIATELY to add weight and your boa will soon do better. Again, pictures will help us help you.
I am glad you will take him to the vet if the mucus doesn't clear up, but please get that t-stat!!!!![]()