Pandora, I assume he's having the bottles because you're not around? If you are at home and want to continue bfing, I would just offer the breast. But if you're out, then I assume your situation is like ours, that the bottle is much easier to feed from and without the feeds through the day the breastmilk situation is getting low. We are ok because P still has at least one night feed, but to settle at night she often requires a bottle top-up.
I guess it's up to you, what you want to do. You could try cutting out a bottle feed during the day to encourage a bigger milk drink when you get home or first thing in the morning. But it may not work.
Personally, I'd think at this age it's ok for them to have less milk feeds as long as they are eating well, and I'd just follow their lead.
Snacks, Im working full time so he is either with DH or at daycare during the day, otherwise Id still be b/f him at those feeds as well.
Im not fussed which feeds he has milk while its not affecting his solids, im just not sure of how much hes getting - he has 180-230ml per bottle, so if its at the lower end that means 360ml is possible all the milk hes getting in to him if he refuses the b/f and so I wonder that thsi is not enough?

Its hard that hes soo different to DS1, who didnt drop his last feed til he was to and fed to sleep every night *sigh*