Did you see nynaeve's link on 3 year olds? It is a secret, but it applies here, too.
46 Reasons My Three Year Old Might be Freaking Out
Sometimes, we just don't know what the hell is wrong with them. It will pass.
I know it is cold out, but can you take a super short walk or just get outside for a few minutes, even if he is fussing in your arms? I wasn't very good at doing this, but even if it didn't make a kid feel better, the fresh air did me a world of good.