When you say cracks do you mean in his central sulcus? If I've got ang concerns about thrush I clean the hoof out and then scrub with dilute Milton to kill any nasties.
The flaking on the soles is normal, they flake away naturally just as skin does, the central sulcus in the middle of his frog does look rather deep and could be hiding a few bugs but it seems to be packed with mud so is hard to see how deep it is.
You can usually tell thrush by the disgusting smell and a black tar like discharge from the clefts of the frogs.
Not sure re the cracks but can see no evidence of thrush form your photo x