Cute 'lil Guy!
Wonder if he is constipated. I'd work on hydration, if he's not drinking on his own, then tube or syringing fluids into him will help. Offer wet soupy feed.
I understand about having different ages within a flock, I do too.
I feed an All Flock feed which can be given to...
He's 11weeks old, feed only the Chick Starter. Layer feed had to much calcium for growing chicks. Possible this could contribute to the mobility issues.
I'd focus on Chick starter and give 1/4 tablet B-Complex daily. Egg as a treat if one is necessary.
When you have time, video of...