Stretching is normal when they are standing up. It is also normal for a chick to stretch while lying down in the brooder. As long as the chick is not under any stress like over heating with panting or being ill in some way, than this is completely normal. Check your brooder temp for proper heat level. Lower the heat if they are over heating.
Good luck with your new babies and if you have any other questions, feel free to ask. Welcome to our flock!
Hello and Welcome To BYC! X4, chickens will stretch their legs and wings, as long as he is eating, drinking and acting normally like the other chicks, I wouldn't worry about it... if what he is doing is different, or if you don't have other chicks to compare to, you could take pictures of what you are seeing and post them for people to look at.