B.,
I haven't read any other posts yet, but my experience is that children often do not reliably sleep through the night until they are about three. Some do, some don't, and it's always frustrating when everyone else's kid but yours sleeps for 8 hours straight!
From your post, it could very well be that your son is hungry at that first wake up--at this point, I would load him up on something filling. Would you consider bringing him to your bed at that point, to soothe him through the next "wake up" without getting both of you out of bed? (My son, 20 months, still wakes every so often, and I am so glad we are cosleeping. But it's still annoying!) If you can, get up with him at 6:30 and keep him up until ten or so, then let him nap. Chances are he'll take a longer nap then. I think I'd go nuts if my son was taking all these little cat naps. The point is to consolidate his morning nap to one big one.
About you: I know how it feels not to have enough rest. It truly sucks, and sucks the life out of me. Take the best care of yourself you can. If it's possible, have your husband put your daughter to bed and go to bed early with your son. (Explain to him that this is only temporary, but that you need your sleep.) Dishes, laundry...all of those things that keep us up can wait! If you can, hire some child care or ask a friend or relative to come and spend time with your daughter while you and your son nap. If that isn't an option, don't feel guilty about dozing with your son while your daughter watches a video or Sesame Street. Really. Two kids can be exhausting!
Above all, try to remember that this is a season. A tiring, hard season, but it's not permanent. My best to you, from one tired mom to another!