I'm not going to be able to help with the poop and vomit issues, but I can say that my dd also arched her back from a young age whenever she does not get her way or feel a bit ... well ... unreasonable. Your lucky that he is not stomping his little feet and flapping the little arms yet .. now that has me totally off my usualy self-controled high-horse , iykwim.

So all I wanted to say is that you are not alone with an arching baby I have a little half-moon daugher myself. I usually just let her lay on the floor for about a second or two, then pick her up and give her something else to do. If she arch while I'm trying to put her in her carseat ... I firmly tell her no and place her with determination into her seat and buckel her before she can slip off. If she throw an arch when I need to pick her up at the shop or somewhere, I just pick her up and hold on tight and say loudly "NO" and just keep walking and thus affirming my autority so to speak. It's hard and she really gets to me sometimes, but it's important to not let them see that you are thrown by their little display of rebellion. Ususally she gets that she is not getting away with the behaviour and stop just as fast as she started .. other times it's tears from dd and heart-shattering-insides for mommy, but after a good cry and a even better cuddle ... all is well in mommy-dd-land

GOODLUCK!!