Formula baby drinking breast milk

My sons 4 months. I bf him for a little over a month n stored some milk in the freezer. Today I decided I should probably give him the rest of it so I can have more room in my freezer. Well I gave him his first bottle of breast milk since we stopped breastfeeding and he was like choking on it? Feeding was very difficult. We had to keep stopping. Is the milk too thin?? Or does he need to get used to the taste again? He drank it in his normal bottle we use so it can’t be his bottle. I really don’t want to give it to him if he’s gonna continue to choke on it... so what should I do?