One option is at that age to up the solids a little. He sounds like a big boy and can probably handle some grain or veggies, which might provide him a "Stick to his ribs effect". Good luck! Of course, now that my lo is 9.5mo, I send some grain, yogurt, fruit and veggie, PLUS 16oz formula.
When I first sent my dd, she was a little younger, but was bf AND formula fed, so I sent two 8oz bottles for the day. Sometimes she didn't finish them, but at least she had and didn't need instead of needed and didn't have...
-Anita