My baby has white patches in his skin after his rashes healed. Does anyone know why it is?

Postinflammatory. It's called postinflammatory change, and it's quite common. After a rash heals, many people are left with discoloration (either lighter or darker than normal) in the involved areas. It is of no consequence, and the color will return to normal with time. A bit of cautious sun exposure will speed the process, but a baby's skin is prone to sunburn, so be very careful.