I've started solids at all different times in that 4-6 month range. Graham I started at 4 months. Ebby had lots of stomach issues and I waited and started her at 6 months. Meghan was in the middle, at 5 months. I had minor troubles with all of them, but every problem was caused by my going to fast with introducing things. So, if you decide to go ahead and start solids, I would go very, very slowly. Offer very well pureed stage one foods mixed with a little of his milk. Start with just a tiny bit of something mild like pears.
As far as amounts of formula, at 5 months Meghan was taking around 25 oz a day. But like pp said, it just depends on the baby. She has never taken the amount she is "supposed" to take.