Hi OP, it's great that the TT has been diagnosed and snipped!
IF you want to continue breastfeeding, then many of us have already given the same advice. Feed, feed, feed. Skin-to-skin, even if it's just lying down for a feed with your top off at nap time and bed time.
I'm sorry I can't recall - has a health professional raised concerns about his weight? Or is it that you don't feel he's gaining 'enough'?
It's true that FF babies can look chubbier than BF babies, but this by no means signifies that they're healthier or happier.
I definitely would NOT be giving formula first, then topping up with breastmilk, then pumping, if my ultimate goal was to happily breastfeed. Cut out the ff, breastfeed FIRST, then if you want to pump you could (tho I never found it v easy to get v much via a pump.)
The only way to boost your supply is to increase the demand from your baby. HTH.