I would start with 5 oz of formula if that is what he takes with ebm now...if you find he is draining the bottles that is a sign you need to up the amount. How old is he? DD3 is 7 months and we mix feed, she takes between 5-6 oz a feed, the odd time 7-8 oz.
As to which feed...we did the bedtime feed first for no particular reason!