I breastfed both of my girls (they never even tasted formula!), and introduced solids (cereal) at about 5 months with both of them, but by that time they were already sleeping at least 6 hour stretches at night. All of the kids in our extended family (except one super premie)were sleeping 6 hour stretches at night by about 4 months and all were exclusively breastfed so I'm not sure about the idea that breast milk won't hold them. By 8-9 months my girls were eating on a 3.5-4 hour routine during the day (solids first followed imediately by nursing)and sleeping two 6 to 7 hour stretches at night with a short nurse in between. I think maybe nursing through the night is a habit. Just think, if you ate lunch at noon everyday for a week then one day you had to wait until 1pm, you'd still be hungry at noon. Same for babies. He really is hungry every 2-3 hours because that is when he is used to being fed. Also, by having a gap between his solids and daytime nursings, he is used to 'snack size eating'. To begin dropping night time feeds, just start cutting them back. Start with the 3am. When he wakes limit his nursing time to 5 mins each side for a 4-5 days, then 2-3mins, then just do one side. His body will adjust to getting his calories at other times so don't worry about that. I assume you are not changing his diaper at night (unless poopy), but if you are stop. Night feeds should be as dark, quiet, and short as possible. Just a little 'warm up' for the tummy to get them back to sleep. After a week of one sided nursing wait 5 mins before going to him. He may start to settle himself. If you need to go in, don't pick him up, make sure he's covered and warm, softly tell him he's ok and walk back out. Wait 5-10 mins to see if he settles. He should be sleeping a good chunk at night within a few weeks. :o)
As far as amount of daytime eating. My girls ate a 1/2 jar of fruit or veggie, mixed with cereal to make about a full jar worth in volume + some scrambled egg or yogurt, etc. at each day time feed by about 7-8 months. Roughly 1.5 to 2 jars in volume of solids before each nursing. Other foods you can try are mashed potatoes with cheese, any soft fruit or veggie smashed up (pears, sweetpotatoes, broccoli, banana,peas, etc.), explore finger foods like cheerios or small pieces of toast. Have fun! Just remember to wait at least 4 days between new foods, so you can identify any sensitivites.
Have fun! This is a great age. :o)