Honestly, it sounds like oversupply or overactive letdown to me (and no one else has mentioned that yet). Do you always switch sides during a feeding? It's possible that, especially after nursing 7 other babies, your body has started to really produce a lot of milk.
Check out this link, and you may want to only nurse on one breast per feeding and start block feeding. It should help your child get more hind milk (which may help him gain weight), and it should reduce the spitting. (I love www.kellymom.com for breastfeeding issues!)
I nursed my oldest for 14 months, my middle child for 2 1/2 years, and I'm still nursing my 21-month-old. I had serious oversupply issues with my 2nd child, and she was a huge, projectile spitter. Once I finally got my supply regulated, she started to do much better. I was more proactive in preventing the oversupply with baby #3, and he did much better from the beginning.
Good luck to you, and congrats on your big family!