Physically, I have all of his vitals and data recorded back at my Fortress. Beyond that, Kon has a lot of emotional grief that he needs to work his way through and there's no test or forced amount of exposure to the sun that will ever be able to hand him an instant fix to that.
He needs support from his friends and he needs the knowledge that we aren't going to abandon him. With the Seal being as unpredictable as it is, we can't provide him with the latter. That's why I think that he needs to accept the fact that we're all not going to be here forever. It's not something that any one of us want to think about, but it's a hard truth. Without it, we'll all suffer more and more every time one of us is taken away.