What about it is he freaking out about?
The laying down?
The taking off the diaper?
The wiping?
Or all of the above.
Could it be that he is one of those kids that is "emotionally attatched" to the contents of his diaper?
Is it that he feels like he has no "control" of the whole process?
Or have there been issues with constipation, rashes, etc that he could be remembering and linking to pain when the diaper was changed...
Is this a new thing or has he had issues with this for a while...
Oh, if only kids wern't so confusing and each came with their own model specific manual. lol
Maybe try changing the peepee diapers standing up...Even the poopy ones if your willing to experiment... Just make him bend over and try to close like a book to clean him up..