My 17 month is teething, while brushing, I noticed some white stuff behind his teeth and he's not eating as he usually would, what should I do?

See your dentist. The "white stuff " could be food or thrush does this smell? Is he having pain? What color are his gums these things need to be known..I would take him to a pedodontist in your area for a checkup.
See a dentist. Preferably apediatric dentist. If your child went for 1st dental ck-up @ age 1 your dentist has a base-line. If not, it's time to have your child examined. Could be anything from accumulated food debris and bacteria to a localized infection. Let a pro take a look and advise you.
White stuff. The white stuff can be undigested food, thrush, or possibly a viral illness.