It explicitly says that he can't die of old age, but that he is injured as normal. If he's not unaging, this means when he reaches 5 decrepitude he becomes bedridden, but he won't die. As time continues to advance, he will continue to gain aging points, which means his stats will continue to fall, and his decrepitude will continue to go up. In Greek legend, if you gain enough decrepitude, you turn into a cicada. en.wikipedia.org/wiki/Tithonus I have no idea if that's current in 1220.
But you were smart and made him unaging, so while it might be amusing to have more decrepitude than Flambeau has Ignem, it doesn't affect him.
Also, being trapped in a box, he will suffer deprivation as normal, which means he will first lose all fatigue, then suffer a light wound, which will advance to a medium wound, and so on until he has a single incapacitating wound. No, I lie, three incapacitating wounds, one for air, one for food, one for water (that is, if he's at sea. Bury him in a lake and he has plenty of water to drink). He makes two recovery rolls each day to avoid dying, which he cannot fail, because of his death prophecy. Every time he fails to get better, though, he gets -1 on the next roll. If he has a decent stamina he will regularly make rolls of 9+, his three incapitating wounds will become heavy wounds, and the penalty to his recovery rolls will be reset to 0. But in 500 years he's going to blow it eventually, and let a penalty develop to his recovery rolls that he simply cannot overcome by himself. Like, -100,000 to his roll. But like Miracle Max says, there's mostly dead and there's all dead. There are plenty of ways to heal a wound without having to roll recovery for it. For example, a CrCo ritual, or a demon who's been waiting for an opportunity to make him an offer. Or maybe a fairy glamours him into health long enough for him to cast his own crco ritual, just for giggles.
If you really need him to throw off his casket and rise up on his own, there's "Greater Immunity (Deprivation)" from RoP: M, but I don't have that book, I'm just seeing it in the unified virtue/flaw index. The downside to that is he will be horribly, horribly awake while underwater, which WILL drive him insane.