Personally, I'd give him more formula, preferably during the day, so that he may sleep in past 5am. My 9 month old has up to a litre of formula day plus three good sized meals.
We have been teaching my lo to sign for a while and he has just picked up how to ask for milk. There have been times when I would never have dreamed of offering him any extra, such as after his meals, so was amazed that he actually drank a few more ounces after he signed for it.
I think they know more about their needs than we give them credit for! :wink:
So I think it would be ok to give him a little more if he'll take it. Could increase his solids too, maybe ?