It
could be that the windfall's were starting to ferment ? So he would be a little bit drunk maybe ?
If it was meningitis you'd be expecting shivering, eyes rolling and for him to be down (rather than standing) could be you've caught it early and that's why it's not as advanced
Or it could be salt poisoning (similar signs) if you've had it hot and he's been without water.
We'd a scare a few weeks ago with a young Kune gilt - she just seemed 'vacant' and a little bit wobbly. We'd run out of feed and had to buy a bag of Farmgate stuff instead of their usual, she'd only had one meal from the bag but after seeing her (she's the smallest of all our pigs, so would have been the first to be affected I think) the rest went in the bin and they got apples until the feed delivery arrived (only 24 hours so not too bad) she was right as rain after about 6 hours.
Hope he perks up in the morning - let us know how you get on.
Karen