Donated breastmilk

So my son is 2 weeks old and we've always had trouble eating. I'm the hospital he didn't eat for the first 24 hours he was born. breastfeeding did not come naturally to us, so we ended up having to supplement with formula. When we left the hospital, he finally latched and he was exclusively breastfed for the first week. Unfortunately after a week, it went back to him not latching, I was only producing on one side, all he was doing was crying or nursing and I decided with a heavy heart that maybe breastfeeding just wasn't for us. He's 100% on formula now and is growing like a weed!

Well anyway, I went to my best friends house yesterday and her sister in law is still pumping and giving her son breastmilk at 8 months old. Well she's an over supplier so she had freezers full of breastmilk and ended up giving me some. Now the next morning I'm kind of hesitant to use it. Is that weird? Is it gonna mess up his tummy, switching back to breast milk or his feedings or anything? He usually drinks about 3-4 oz of formula every 3 hours. I really wanna give it to him but now I'm just afraid it's gonna mess us up. What do you think?