S.F.
I'm going to offer an answer from another point of view. Please forgive my opening question, as it will probably come off as a little too blunt.
Who are you nursing for? You...or your son?
Please think carefully about your answer. If you are of the philosophy of following your child's lead as to what he needs or doesn't need, then my next question is why is this such a huge issue? Your son has let you know, loud and clear, that he's ready to be done nursing. Formula for 3-4 months until your pedi says he can have milk isn't the end of the world.
However, if you're nursing to one year because that's what "everyone" says is the "right" thing to do, even though your son is done, you're tired of pumping, etc., then I would say the issue is yours.
It has been my experience--and I know I'll be totally slammed by the other mamas for saying this--that while nursing is the ideal, formula fed babies are just fine in the long run. I had flat-inverted nipples and nursing was a horrible experience. That was with lactation consultants, working with La Leche, etc. I squeaked four months in with my first son. I gave it a go with my second son and he flat out refused to nurse...again, I had tried everything available to me, including pumping. I finally gave up and gave them enfamil...the same stuff my mom gave me and my sister 25 years earlier. How did I get them to drink it? I just gave it to them as if it were the most normal thing in the world. They were hungry, so they took it.
Guess what? At 12 and 16, my sons are just fine. They're smart, they're funny, they enjoy athletics, they are completely fine and normal young men.
I had my babies in the mid to late '90s. Most of the moms I met through various mom/baby functions were not nursing for the health benefits, but for bragging rights. "Oh! You only nursed for THREE months?! I nursed my precious Julie Anne until she was two! You've never met a healthier, better behaved 2 year old! Julie Anne! Stop playing in the street..." The best moms I ever met--the ones with happy, well adjusted ADULT children, were the ones who learned when to do what's right for their family, despite what "everyone" else said was "right."
Just some food for thought.
Good luck.