I was going to say it sounds like he may be stocked up; however, if I'm remembering right, you don't stall your horses? So that is probably not it, if he has room to roam.
When he had his puncture wound, were you able to get any kind of antibiotic into him? Not to scare you, but it 'could' be infection, stemming from that original wound. Not the likely cause of his swelling, but certainly a possibility.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."