Don't worry about a 15-month-old supposedly having flat feet. At this age, you are supposed to be letting your kid walk around barefoot so his muscles will develop. Who is "dey" who told you he had flat feet? If it was a shoe salesman, was he trying to sell you something? Talk to your baby's doctor at his next appointment, to see if there really is anything to what dey told you.
I have flat feet and its terrible. My back always hurts and my feet do to. I usually only wear flip flips because that's the only thing I personally can wear and feel comfortable.
I have flat feet its horrible sometimes cuz it affects my back but i put these things in my shoes with an arch and that helps me but idk how small they make them? story i don't know what they are call just in Spanish