My 14months son has white stains on his top teeth. He sucks his thumb. He 'brushes' them without toothpaste. Is this normal?

Not normal. If the teeth did not come in this way, then this is the start of tooth decay. If they did come in this way then they are malformed. Regardless, you need to have your child evaluated by a pediatric dentist. Also you should be brushing their teeth for them. A 14 month old is not skilled enough to brush their own teeth. Thumb sucking will not cause stains but can change his bite.
No. 'white stains' may indicate some illness your child had when teeth were being made as an infant .. Stains may be part of your child's teeth.
Long time stains? Did your son's teeth come in that way or did they just appear? They could be areas of "hypo calcification, " meaning not enough calcium in his diet. Is he drinking milk? Did your or his father's teeth at this age look like this? Could also be genetic. It is time for your son to go to the dentist for a routine checkup to make sure all is well and his teeth are coming in right.