I wouldn't be to worried about it. They use a hyobranchial pump, like fish, to inhale the food because they don't have tongues. Often times the frogs just get things in there the wrong way. They will pump it out and then eat it again. I have seen it happen plenty of times with foods they knowingly love.

You are doing a great job taking care of them with water changes, care, feedings. Sometimes they are just like kids. They just chuck it out. :P If you get over concerned about the little guy you can always get him his own tank. I wouldn't think you are in that kind of situation at all. I know one of my frogs eats less than the others. One eats more. *shrugs* they do what they do. I had one male who sang all night and one who never did. When one passed away it seems I am left with the one who doesn't sing. *shrugs*