There should be enough for him, as baby's (or breast pump's) demand dictates supply. I imagine he would've drained your breasts when he BF (babies are much more efficient than pumps!), and if you'd have put him back on the breast instead of giving him a bottle 1.5hr later, there likely would've been enough for him then.
Some babies snack and have littler feeds and are on the breast every 1-2hrs, whereas some take bigger feeds and go 4-5hrs. All depends on the baby, but regardless, your body should adapt.
When you're expressing though, try to help your letdown reflex by having a blanket that smells of him, looking at a picture of him, etc. Sounds silly, but it works (all to do with the baby being much more efficient than a pump in loads of ways).
Any other mums have opinions on alternating breast and expressing?