My 3 month old has a thick white coating on his tongue which gradually formed grey spots. Looks like mold! Thrush or molding milk?

Lactic acid. Bottle feeding can build up residue of Lactic Acid on tongue from milk looks like mold wipe or brush off.
Normal. Having a white coating on the tongue of in infant is not abnormal. The taste buds on tongue can grow out to look thicker, taller, or hairlike. On a child, the grow is quick, the diet is soft, and there is little action of tongue to remove this growth. The diet of formula or breast milk can be responsible for the staining with some slight color variations like gray. Not to worry. Discuss with pedo.