You won't have to worry about him skinning his knees (since, you know, no skin) in this boy's green skeleton skater costume.  The hoodie (which he'll totally want to wear after Halloween) is emblazoned with flaming skulls that almost match the gruesome skull mask, while the green skeleton-hand gloves round out the illusion. He'll think it's sick (which to him is a good thing.) Use some costume makeup to green up whatever healthy pink skin is still showing.