Your supply has not dropped. At 9 months, your body is expert at making only waht your baby needs, and it won't produce a massive letdown, nor will you feel full. Also at this age, since he is nursingless, you body is only making the higher fat milk and much much less of the foremilk that made you feel so full before.
Basically right now, if he nurses for only 10 minutes, he is getting all the fat, nutrients, antibodies, etc that he needs from your milk. The less he nurses, hte more concentrated it becomes. Just keep nursing him.
I agree with Annette, that intro'ing formula right now is not necessary at all. Your milk supply is fine, its just more concentrated and your body knows what its doing now perfectly.
What you pump is not an indication of what your supply is, your baby can nurse out at least 2oz more than what you are pumping!
So keep nursing him, he sounds like he's right on track. At 9 months both my boys were eating table foods and nursing 3-4times a day. Don't owrry about how long or how much he's eating, if he's hungry he'll let you know. Again your milk supply is not low, its more concentrated now, heavier and fattier all at once so that is why you aren't feeling full. Your body isn't making a huge excess anymore, and your letdown has controlled to his taking.