is it muddy when he is turned out? Did the swelling go down after you exercised him? How long has his leg been swollen?
If it is muddy and he's been standing in the mud, he could have mud fever. My mare had that and from what I read online, if the leg starts to swell, you need to call a vet.
If the swelling went down after you exercised him, he might have been "stocked up" which happens when a horse is standing for a while and excess fluid stays there (i think anyways)