Almost out of breast milk!

Hey all so I was exclusively pumping and giving my son breast milk only he is almost 9 months. Well I am 12 weeks pregnant and once I got pregnant my supply dropped drastically I had a lot stored but now I only have a few weeks left of milk. Should I start doing half breast milk and half formula? Or just give him all the breast milk until it’s gone and just switch to formula for the last 3 months. Idk what to do!! Any suggestions or help please! Thanks in advance!