D.W.
While it could be a growth spurt that sounds like an awful lot of formula. Take that with a grain of salt since I only nursed mine but it sounds like a lot. Make sure you are only using a slow flow nipple. I'm thinking at his age it should be somewhere around 3 oz max per feeding but hopefully some formula moms can offer up better info on that.
also, why would you think you couldn't keep up with him? Because he was nursing often maybe? That's what breastfed babies do. Especially in the first 6 weeks. It gets your supply built and regulated to what baby needs. If you want to continue to offer him breastmilk. Nursing every 1.5-3 hours is well within normal range. And you start timing once you *start* the nursing session not once you finish. It can seem as though some babies want to nurse all the time in the beginning and to someone who may not know they automatically think baby isn't getting enough. I highly recommend putting baby to breast when you are with him-morning before work, once you get home, anytime he needs to nurse at night and on the weekends. That way you'll keep a supply going for this next year since baby is best at getting and stimulating breast milk production. Congratulations on the baby!