This absolutely sucks. I bet he doesn't play football again.
When they found the clots in February, he ceased all football activity for 6 months and went on anticoagulants. When he came back in August for football activities, I'm assuming you can't take the anticoagulants and play, so I'm assuming he was off the medication. If he's been off the medication and they've returned, that absolutely sucks. I'm no doctor but blood clots seem like something that can be managed and treated with no impact to his quality of life, but football could be over for him.