They usually indicate they are ready to extend their feeds when their bottles decrease not increase! What you will find is that they are not interested in a feed or will only take a small feed bacuse they aren't properly hungry.
I don't think his bottles are getting too big, if he is taking 5oz and draining the bottles then he needs a bit more in them. They are doing a huge amount of growing in these early months and do need the calories, what you will find is that he may increase his bottles to 6oz+ but it will even out. If I remember right my babies both steadily increased their intake up until they got to 7/8oz every 3 hours and then they started to be able to go a bit longer between feeds.
I wouldn't increase the time between feeds just yet, he won't be able to do the A time he needs for a 3.5 hour EASY and it just makes everything harder. Babies usually transition to a 3.5/4 hour EASY by 4 months, some bottle fed babies do need to go to 3.5 hour EASY around 3 months, but you are a few weeks shy of that.
HTH!
Laura
