ADFs are extremely sensitive to water quality and changes in water parameters. Shedding can be a sign that your water is not as clean as it should be, or that something has drastically changed since your last water change, such as the source, or additives. I would do a large water change and get a sample of the old water tested either using a master test kit or by taking a water sample to the pet store for a free test.
Frogs simply do not tolerate high nitrates or any amount of ammonia, and they need to be acclimated to new water very slowly and gently. They are also sensitive to salt, copper sulfate, and any kind of medication.
They are also temperature sensitive, so you need to keep the temperature of the tank in the betta's low range, around 76 degrees--definitely not around 80 degrees or higher.
Can you tell me more about the size of the tank, the temp, frequency of changes, and how/what you're feeding him? Is this the only ADF in the tank?