Is it ok to warm up my sons milk in a plastic bottle in a microwave?

I highly recommend you do not because even if the plastic is microwave safe, studies have shown that microwave radiation can affect the milk in the bottle. The intensity of the radiation is pretty high in microwaves and it could affect baby in the long run. I would recommend an electric bottle warmer but If you don't have one of those, you can use a jug or container of warm water. Place bottle of formula in the jug for no more than fifteen minutes. Any longer and bacteria will apparently occur. Once the bottle is warm enough, give it a shake to ensure that the temperature is even.