Give him a wide variety of weeds such as dandelion greens and flowers, hibiscus, pansies , chicory , daisies , sowthistle , hawkbits , sheperds purse , clover and you can find more on http://www.tlady.clara.net/TortGuide/diet.htm#plantlist and they show some pictures so you can see what the weeds look like
I would say it's probably diet related, rather then humidity related. Do you feed him much aloe or squash/pumpkin? These foods can cause runny poop. More fiber will help harden it.
When tortoises are fed a diet of mostly store-bought greens, their poop is a bit on the runny side. Try to add more clean plants from outside to his diet.
Different species but my DTs had this problem when I was feeding a lot of store greens. Upped the weeds/grass and laid off the greens for a week and they regulated. Now I feed a lower portion of greens. Incidentally, when I first tried to get them to eat grasses they turned up their tort noses. After a couple days of just weeds and grasses in the bowl they got wise.
The most likely cause of trunny poop is insufficient fiber in the diet.
You might have to be tough here, but it is for yuor torts own good.
Chopp the loved lettuce finely and mix in some weeds and leaves that have more fiber. serve the food wet so the best bits can't be picked out. he may well resist eating at all for a few days, but he will give in and t won't do any harm.