Hi emer,

Sunshine and Heather are right , he needs to be remove into a quarantine tank. For the sake of all your frogs.
I would breakdown their existing tank , sterilize it and everything in it as well ! If you have been considering a new enclosure, this might be a great time to get it !
Boil all the deco . If there are objects that can not withstand boiling ,soak/rinse in a very weak bleach solution ( rinse, rinse, rinse) than throw them away .

If you are comfortable with it, you could post their tank size and a description of your cleaning habits.. etc. This information may help determine how to tweak your efforts in caring for them.

Here is information for a vet in the US you ,may be , able to speak to via email. He is very caring.
At the very least, he will point you in the right direction as your frog does need a vet. He will get better with the proper medication.
http://www.frogforum.net/tree-frogs/...-prepared.html

How old do you think he is?

You could try a honey bath. The honey has some anti-fungal properties. However this is NOT a cure!
Frog Forum - Basic Frog First Aid

Hope this helps
Lynn