A mouthful of substrate will not cause an impaction without other factors playing into the situation. Does his tank reach the low 80s during the day? How long has it been since he last pooped? His legs might be stretched back because he's so full of waste, it's pressing on some nerves (I've seen it happen to a severely constipated dog, may not apply to frogs).

You need to take him to a vet. He sounds completely obstructed, so a vet will likely have to manually remove whatever is blocking his digestive tract. I wish this wasn't the case, but it's likely the only choice you will have, so I hope you have a vet or are able to find one.