First thing to do is check to make sure you're roof is on correctly. If he's in the house and still suffering, then this is likely the problem. And just in case, go up one level (if there's not another floor up there already) and cover the rooms with the free floor tiles, just to be safe.
A vampire can stay in sunlight for a maximum of 3 hours. But when you exceed that, you have serious problems. First thing, you will pass out where you're standing, and your thirst will plummet. You won't be out long, but when you come to, he's going to have a vicious -50 moodlet called "Too Much Sun," and will probably need to see to his thirst immediately. It lasts 2 days (so long as you don't go back out in the sun at all), and while it persists his thirst will constantly decrease at an extremely rapid pace. During this time, you have to watch him extremely carefully, because the instant his thirst hits 0, he will die. From my only experience with it, this need to constantly drink plasma juice will not permit him to get a full night's (or day's) sleep. It just falls too quickly.
And windows block sunlight, even without curtains.
