I sounds like you want to make some pretty drastic changes from what he is used to. I would advise against changing from tub to terrarium in the first 90 days unless he is ridiculously cramped. The smaller and more simple the setup, the easier it will be to notice disease. If you move him, and he starts behaving drastically different, is it because he is ill or because of his new surroundings that he just needs to get used to? Generally a brand new snake gets a brand new enclosure when it is purchased or adopted right from the start, but people who have some experience will generally tell you, it's better to start with a simple tub setup for quarantine and move to a roomy enclosure later on.
If the humidity is a problem, what are you using for substrate and how big is his water dish? What is the humidity like outside his tub?