First I would arm yourself with any statutory info there is for where you live (depending on that there may be nothing in law or quite a lot which tells employers what they have to do to accomodate you).
Then I'd open a conversation with him about breastfeeding...something like "I dont know how supportive you are of breastfeeding, how much you know about how breastfeeding works, or the benefits?" Based on his answers you will know how much you need to fight and educate him or actually if you just tell him what you want and he'll be fine with it.
I think a few good points to make are:
1) Pumping often enough will reduce the chance of Mastitis, which if it happened could mean lots of (possibly paid) time off work while you recover.
2) The longer your child receives breastmilk, the less likely they are to get serious illnesses or be hospitalised before the age of 5 (which again would be extended periods off work).
Depending how old your LO is and what childcare you have arranged, it might be worth considering if it would actually be quicker for one of those sessions to have someone bring LO in to work for you to feed, or if at lunchtime you could leave work to feed her.
I wouldn't actually mention making up the time unless your boss asks you to.