Do u mean he is older than 10 weeks? If so, you're really, really, really, late in helping fix this poor chick.
It is referred to as Splay leg, or some variation there of depending on who u talk to. If u catch it when the chick is very young it is very easy to correct by hobbling the chicks legs together, or otherwise finding a way to keep them together correctly under the chick, there r tons of ways people go about it depending on the size of the bird & what they have available. Correcting the substrate the chick is being kept on is a must as well- too many people try to just use newspaper or paper towels which offer no support. If it is not the substrate, it could be trauma, or sometimes a lack of calcium in the chicks diet or other medical issue is the cause.
Once the chick gets older correcting the problem completely gets harder.The longer a chick is left with splay legs, the longer they have to calcify that way. Once a chick is full grown (very short time), it can be so bad that only surgery can be a fix.
I've included a link that is got some good basic advice, & ideas with pictures on how to hobble or use a sponge to keep the birds legs in place.