It's great you're working with a lactation consultant. You may have a correct latch and still have pain, however. If your son is tongue-tied you will still have extreme pain, even with correct positioning, etc. I had this problem with both my boys and had both of theirs cut (Caleb's at 18 mos and Keenan's at 1 mo). I myself was tongue tied until I was 17 when I chose to have it severed. I knew when my first son was born (in the delivery room) that he was tongue tied but was convinced by the hospital's pediatrician that he wasn't. Make sure there's nothing physically wrong with Duncan's mouth. If there's nothing wrong, I suggest using nipple shields until the cracking heals. The breastmilk on the nipples works also. I also read somewhere about putting lettuce or cabbage leaves on them. You'll have to check on that as I can't remember the vegetable! :) If worse comes to worse, just pump and bottle feed. You'll have the peace of mind that Duncan's still being nourished by your body.
Good luck, I really relate to your situation. It's so frustrating, but there is help out there. Don't give up!
C.