My 21 month old is walking on the insides of his feet and has been for months now. I'm not sure if I should be concerned with this or if it is something that he will grow out of. He even walks this way in shoes. All his shoes are worn on the insides and pretty new on the outsides of his feet. Should I be concerned or wait it out?

They make corrective shoes for babies and it may be best for him to have some.
It could become a more serious problem when he is older and he may go through way more shoes in a life time than the average person does so in the long run it could save money as well as help him.
My nephew had to wear corrective shoes the first couple years of his life for the same reason. His feet are fine now.