If he really weighs in at 12 lbs at 5 mo age, I think he can be only be a Cornish X. If so, you would be doing him a favor by sending him to freezer camp right now. They are meat birds and are meant to be butchered at a youg age. If you let them continue to grow they get grotesquely huge and it is just cruel to let them get to continue on at such massive size. They get to the point to where they can't get on the roost and have a hard time even standing.
He's a Cornish X cockerel. I agree with Lbrad7, that the most merciful thing for him would be to butcher him. The other white chicken behind him looks like a Cornish X pullet. Many people have kept the Cornish X pullets for layers. They are pretty good layers, but because they are large and can't move as fast they are usually on the bottom of the pecking order, and are often "over roostered." They are also more subject to predation, for the same reasons I mentioned, and also because they are white.
Oh you can grow them out. Look at Godzilla. He was over 21 pounds at one point. This is a cornish X. You may have a rock. He was able to do everything including kick holes in 5 gallon buckets so I don't think you can say he was immobile. Good Luck!