I couldn't read and not respond.
How many times has your son done this? Is it every nap or has it only happened a couple times?
Firstly, I'd lay off the spanking thing. I can't think of a more rediculous (and humiliating for your son) way to teach a 2 year old how to use the potty and not play in his "creation"
Secondly, There are no rules for sleeping. If your son feels well rested and doesn't seem tired (cranky/unruly etc) that even if he took a 30 minute nap, it would be sufficient. It sounds like your dh is the one who needs the 2+ hour nap. Many 2.5 year olds aren't even taking naps anymore.
Lastly, could you try letting him nap in the same room as you, or in the hallway? Is he somewhere that when he wakes he can come and get you? Can you get a video monitor so you can watch for when he wakes up.
Even though it's completely disgusting to us, the idea of playing in our own feces, that's really a learned thing. To your 2 year old, he's probably thinking "wow...I made that" combined with "that has an interesting odor" combined with "that feels neat" (now, you can't tell me you never enjoyed playing with mud!
If I were you, I'd really just try to catch him upon waking so he's not alone to do that. When it does happen, just remind him that it's not ok to play in his poop and help him get clean. In my opinion, don't shame the poor kid, he really doesn't know that what he's doing is gross to you and isn't capeable of entering YOUR mindset yet. Even though it probably won't curb his poop-play, you could start doing some real tactile projects with your son, maybe he just needs some of those types of experiences....play-dough, clay, mud, corn-starch and water, fingerpaints etc.etc.
good luck, hang in there, it's common!, this too shall pass.