It could be that he is too heavy now for his bones. Maybe his bedding is too hard? What is your pen on? I would keep him away from anything too hard. That includes concrete, gravel, and packed super hard dirt.
He should have his bedding overtop of dirt. That seems to be the only way they are comfortable. I personally keep my steers on a foot of hog fuel.
He could have been sleeping wrong too, or injured it playing around. My steer sleeps with his front legs straight out, and then is super sore when he gets up. After a bit it leaves though.
I have heard of some people giving their steers joint supplements. They don't sell them where I'm from, so I don't know how they work. Something anti-inflammatory would greatly help him, but I can't think of one that he could have, being so close to his slaughter date.
Make sure to walk him a snails pace in the show ring so that it doesn't look like he can't support his weight too.