My Som Who Was Born At 26 Weeks Is Now

My som who was born at 26 weeks is now 4yrs old but has frequent complants that his feet are really hot and his leg hurt. A few times in the last year after complaining he has lost his balance or not be able to walk normally. And after a few min he is fine
Our pediatrcian is not worried said common for kids should i worry and oush for more test.