My baby keeps pulling at his bits hard everytime we take his nappy off, why?

I have had this with him for the last 3 months, we have taken him to the GP who perscribed some anti-fungal cream. This did not work. i changed his nappy brand, this did not work. I have even taken to bathing him twice a day, this makes it worse. He sits there crushing his bits in the bath until he goes purple. Whats going on? Has anyone else had this with their babies? He is 13mnths old and is so persistant on grabbing them and crushing everything, until its red raw and bleeding.