Hi--
I breastfed through a pregnancy and it is challenging, but not impossible. You'll have to up your water intake considerably. Remember, your uterine water replaces itself every 24 hours and you need water for you own body, so the first thing to go is the breast milk. Taking alfalfa and extra b-vitmains also helps. DO NOT take fenugreek--it is contraindicated during pregnancy. Acupuncture can also help. And you'll need to eat enough protein for you, the baby, and the breast milk. You won't dry up completely, but during the 3rd trimester your milk will become more like colostrum. The taste will change and you probably will make even less, so be prepared for that. Your son might wean himself at that point because the taste change. (Mine didn't, but he was very, very attached!!) I'm not sure about the formula part because my son was old enough when I got pregnant that I didn't have to worry about that part. I know goat's milk is the closest to human milk. Ask about it at Vitamin Cottage--they usually know how to hook you up. Also, you can make your own formula--their are recipes all over the place. I don't know it off the top of my head, but I can get one for you from our natural doc if you like.
Good luck with everything--and congrats on the new baby!
J.