It might be thrush, or it might be eczema. For thrush, try Vagisil cream, and for eczema, try an over-the-counter cortisone cream like Cort-Aid or Cortizone-10. Don't do them both, though. I'd first look up thrush, it's a yeast based infection (and seems kind of likely since he had penicillin). Look at the pictures, and that looks like it, try the Vagisil or any other cream made to kill yeast infections. (It used to work wonders on my baby's bottom when he got a diaper rash that had a yeast-infection foundation.) Give it a few days, and if it does nothing, then try the otc cortisone.