Have you really looked threw the tank for poop? My Azy has been berrying her lately for some odd reason. I found 4 days worth yesterday when I took everything out and dug around a bit. @.@ How dirty is the water dish in the morning? If hes pooping in there then its dissolved and looks just like dirt by morning.

If all of that is ok then try having a large cup of dechloinated water that you've warmed in the microwave for 20-30 seconds. Take room temp dechlorinated water and put a little into a cup that has a top you can put air holes in and place your frog in that then slowly add the warmed water. If you've let the cup you microwaved sit while you caught the frog and put it in the cup then the water should have cooled enough that it shouldn't shock them.

BlueisallIneed (another member) has also said gently massaging there sides while in the warm water can entice them to go.

If you cant get him to go in the next day or two I would get a hold of herp vet and get him in. They can prescribe something like a laxative that will help.

I hope your little one gets better soon.