Yep u can mix it! I had to mix mine last time until I could make enough
You can try mixing a little at a time with his formula and gradually increase the amount each time. It'll get him used to the taste again.
i'd try again too, but i'd try to stick with a routine as far as when you're bottle or bf'ing him, he's probably confused. trust me I know it's hard, my little girl is a week and 5 days and bf, plus I have my 3 1/2 year old and tons of appts and housework, but he may do better with a routine on when you'll give him formula and when breastmilk.
keep trying dont give up yet...breast milk is the best thing