It looks like he has oral thrush. It is caused by an overgrowth of fungus called Candida Albicans. Not to worry though as it is usually nothing serious. If the white residue cannot be rubbed off easily, it may be oral thrush instead of milk residue. Do bring him to the doctor and get an oral antifungal gel. http://sg.theasianparent.com/oral-thrush-in-babies/
Does your baby has fussiness while feeding and the white spots can't be cleared using cloth/gauze, it is oral thrush. This is quite common due to immature immune system of young babies. If you are worried, I suggest you to bring baby to visit PD to get antifungal medicine.
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thanks