I think your son is confused...to be honest, I would be. If my routine was dropped from out of nowhere, and I was expected to do something I wasn't familiar with, then it would do a number on my mood and behavior too.
It's not bad that you got him used to something, but if you're going to change it at all then it should be done gradually or you're going to get a frustrated and confused little person...who is OVERLY tired!!
My son and I have been co-sleepers since he was 2 months old and home from the NICU. But, as he's gotten older and we're moving towards sleeping in a big boy bed, I have been trying to get him 'used' to sleeping without Mommy in the same bed. He falls asleep with me cuddling him, but then I replace me with a snuggly (dino stuffed animal) that he's gotten used to over the last few months. I can't expect him to quit cold turkey, because then it's just going to lead to set backs and frustration on both our parts.
He very well may be moving away from his naps, but most kids that I know, nap well into their school years...my godson is in first grade and still naps for 30 minutes when he gets home.
I understand your frustration, but your little boy needs some understanding and patience while he transitions. Kids can't 'work it out' on their own...especially at this age. It won't take long if you explain it to him, and give him ownership over becoming a big boy. My son loves it when I tell him he's a 'big boy'...but, the key is to not rush it. Let him move at his own pace...try talking to him about what's going to happen.
I have been explaining it to my son, and what I do is I tell him 'Mommy will be here while you fall asleep but, I may not be in bed with you, if you wake up and need me I'll be right on the floor.' And, I'm working up to eventually Mommy will be in her own bed...with him being in his own bed. He does pop up every now and then and ask for me, but I rub his back from where I'm at on the floor or I come from what I'm doing to soothe him if he needs it. But the key for me is to make sure he knows I'm there if he needs me...so, that he can get peace of mind while sleeping. If he still needs me then I'm there, but I cannot just let him figure it out on his own...
It sounds like you've got your hands full with the two little ones and it's awesome that your hubby helps out!! Good for you both for being a team. You might want to include your son as a part of the 'helping with the baby', it will give him a sense of being a big brother. If he really is giving up the nap, then create a quiet time activity for him that will allow you some moments to get things done.
I truly wish you the best of luck!!