Brittsy,
either way, you are doing more than many women have done. Feeding expressed milk is fabulous, even if you have to substitute with formula.
As for mixing it, if you have been doing it and your lo is not complaining, and likes it, I wouldn't change it.
The only idea I have is that if you still want to try, offer your breast when he is sleepy or sleeping. Sleeping baby is usually relaxed and can nurse from breast even when on a nursing strike. I have also heard of taking a bath together and offering breast while in water, some moms here swear by it. If you have even a small success like this, you can continue from there, but if you decided to switch completely to pumping, it is fine, too. It is true that some women can't keep up their supply when only pumping, while some can. It really depends a lot on your body, how it reacts to a pump. I know at least one woman who managed to pump for her LO for 18 months and stopped only because she has had enough.
Good luck!