Sorry
Patsy and Lindsey have given you excellent advice.
You do what to switch substrate..........to cocoa fiber instead.

Please let us know :
WATER! - what water are you using- It must always be de- chlorinated
EXACTLY- which calcium product are you using.

This sound like it could be symptoms of MBD- (even know you have been dusting )
It could be as a result of some food competition?
I wood remove this frog and put it into into own little 'hospital' tank
A seperate take will enable him to eat well and you will be ablt to do treatment soak, monitor food intak and poop.
Feed him well, and daily for a while-- make sure his feeder is not too large
Do the pedi alternated w/ honey soaks for a couple days.
Do the homey soak first so the frog can rinse in the pedi soak.

In the link below ... toward the bottom of the post information about a rescue product for calcium deficiency:
You might get an order placed for this --just to have on hand. Please read all about if first.
You might seriously consider taking this little guy to a herp vet !
Fluker's Liquid Calcium

Instructions for the soaks/quarantine are here:
http://www.frogforum.net/tree-frogs/...-supplies.html

Note:
I see you have your 'country' as Scotland.
In the US plain Pedialyte is a electrolyte replacement used sick children and infants.
Here it is found in any supermarket or pharmacy. I'm not sure of the Brand name of a product like this if you live live Scotland.
It's likely there thought, it's made by Abbott Pharmaceuticals.

Keep in touch-Lynn