Thank you so much! I will put him on paper towels and look into getting a different substrate. Also, the reason I was so surprised that the humidity rose so much is because I've had this setup for a few weeks now, though it is the first time I put water in and ran my UTH for more than an hour.
Also, you shouldn't need to change substrate, I use eco earth and I absolutely love it... I just need to let it dry throughout for a week or two before I use it!