First of all he may be ready to shed. (if not)
Since you have a fifty gallon(i assume it has a screen top), the heat pad may be drying him out and the tank, try placing some solid material(ie. wood, plastic) over the the screen top(allow air in of course).
Or the heat pad is too hot. You have mulch on the bottom of the tank and your snake will become in direct contact with the glass and burn himself.
Try placing multiple hide spots(in different parts of the heat gradiant). Also you may want to differ the sizes. When he choses a size, stick with it and introduce another into the enclosure.
These are a few things to consider..
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"You can't appreciate Shakespeare until you've read him in the original Klingon."