Frozen Milk

I stopped pumping when my son was 8.5 months. I had 2000+ ounces of frozen breastmilk saved so It really wasn’t that big of a deal. The problem is that recently he really doesn’t like the frozen milk and we have to fight to get him to drink more than 1-2 ounces. Meanwhile he will guzzle a 6oz bottle of formula down no problem 😒 He is almost 11 months so he still technically has only 1 month of milk left, but our pediatrician really wants him on breastmilk or formula until he’s 12 months adjusted which is not until the end of October. He also had weight gain issues and had to have fortified milk for awhile so I am really not rushing to get off him milk. Not to mention I spent a lot of time and money pumping and storing all that milk. Is there anything I can do to get him to drink it? or should I just donate it and switch to full formula?