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

It is best not to warm your baby’s milk in a microwave. The milk will continue to heat up despite being removed from the microwave. In addition, uneven pockets of scalding milk may form during the warming process while may increase the chances of babies getting burns to the mouth and throat. Heat from the microwave could also be too intense and may destroy some of the nutrients in the milk. To warm milk, simply place the bottle under lukewarm running water to warm it up. Alternatively, you can also place the sealed container into a bowl of warm water for about 20 mins to bring it to body temperature (around 37 degrees). Avoid using hot water to preserve the full nutritional value of the milk.

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.