Right off hand, I would not be extremely worried, but I would continue to keep a close eye on him. If he is walking around with the others and particularly if you see him eating and drinking, he just may be lazy. He might also have a sore leg or joint. It would not hurt to start a Safeguard round of worming.
My sure fire test of whether or not I should be worried is as follows:
Step 1: Go into the pen. My birds are friendly and will approach me fairly closely, but I cannot easily approach them closer than a few feet. If I begine walking directly towards a bird and it does not get up and move, I have a problem. I will catch it and do an exam. If there is no obvious external injury, it gets the first dose of Tylan and we proceed from there. If it gets up and moves and has no observable limp I proceed to Step 2. If he has a limp, I gnerally will watch it for a few days before I get us both heated up with a net catch.
Step 2: Feed its favorite snack. If it makes a beeline to the food, I consider myself paranoid and continue to watch for a few days. If it does not make a beeline for the food and/or refuses food, it gets caught an examined. I may or may not start antibiotics in this case.
It is funny how we get tuned into our birds health and kind get the tingling sensation that someting might be wrong. I have found, however, that with my main flock, birds just have bad days sometimes and after a couple three days they get better on their own or resolve their own issues. It seems like once a month with as many birds as we have that soemeone has a limp. I have never had to take drastic action. It always clears up. I have always been fortunate that I have never had a bad illness in the main flock.
The birds I buy at auction are totally different, however. It seems like in the first four months or so they have all kinds of issues and can go from fine one day to next to dead the next. With these birds, I almost always start a course of antibiotics now as sson as I get them and worm with safeguard immediately.