My 20 month old son has 2 large white patches inside mouth on his gums, a little bit bigger than a pencil eraser?

Yeast. Loose white patches inside the mouth could signify the presence of yeast commonly known as thrush. The yeast infection can cause problems in other parts of the body, like diaper rash. If you do have thrush, make sure you optimize oral hygiene, if your baby is still on bottles or paci, sterilize them or better yet, wean your baby off them. Consult your doc, as thrush needs a anti fungal medication.
Trauma. Another possibilty is that your child is chewing or rubbing with their molars and it is very irritated. Your pediatrician will know.