My son has this big white patch on his shoulder that a lot lighter then his other skin what could it be?

Could be...: Sometimes fungal infections can make the skin lighter. In order to really diagnosis what the cause is you need an evaluation by a doctor.

Multiple causes: If the patch is singular, has been present since he was small, and has not changed over time, it may represent a hypopigmented birthmark (achromic nevus). If multiple and increasing in size and number over time, it may represent vitiligo. A dermatologist coukd help differentiate the etiologies.