breast milk supply drying up?

So my son is 6 1/2 months old and I was exclusively breastfeeding until 2 months when I had to go to work. he gets formula while I'm away, and then I come home at lunch, breastfeed him and then after work I breastfeed all evening, and 1-2 times during the night. Fast forward to now at 6 1/2 months and I don't know what's wrong. It's like there's never anything in there, and he also just doesn't seem to want to nurse. Whenever I sit down with him to nurse it just ends with both of us frustrated. He latches, and unlatches and latches and unlatches. I get frustrated and continue trying to get him to latch on and he gets super angry and just ends up screaming and crying and I think now it's having an affect on my production. I'm worried. I love nursing and I'm not ready to give it up yet 😔 does anyone have any tips to help?