Julie - My son is also 7 months old (or will be on the 13th) and has even less formula than your little guy. I too wonder if he's getting enough, but he's happy and healthy, so I'm not worrying too much.

Alex is now just under 18lbs and usually only eats 4oz at a feeding. He has eaten more, but rarely more than 6oz. He is eating solids and has been since 4.5 months. Here's what his daily intake looks like (or has for the past week or so... things are always changing) Times are approximate.

Morning (8:30 - he sleeps in): 4oz + cereal
Lunchtime (11:30): 3-4oz + veggies
Afternoon (3:00): 4oz + an occasional additional 2-4oz later in the afternoon
Dinner (6:00): cereal + veggies + fruit
Bedtime (7:30): 3-4oz
Late night (10:30): 2-4oz (my dh feeds him before heading to bed - awake or not)
Early Morning (4:00): 4oz (although, he has slept through this feeding for the past 2 nights - yay!)

He had weight issues early on and we've been keeping a very close eye. Once he weaned and started solids, he caught up VERY quickly.