Weaning off formula

So at first I only breastfeed my boy, but once I got home from hospital my milk supply dropped a little so I used formula as a top up. 9 weeks old now and all of a sudden Bub is pretty much refusing the be bottle fed formula! My supply is definitely still low so it's a little tricky keeping him happy, that's why I was just wondering if it's normal? I haven't changed bottles or formula brand, and I thought if he was going to wean off anything it would be off the boob?