I hope you will take comfort in knowing that the negativity your son may have been exposed to is a weak, superficial force indeed when faced with the loving-kindness born of emptiness. Even loving-kindness that simply reflects emptiness without any realization is more powerful than all the forces of hatred and desire in the entire universe. Please have faith that your son, being of you and intimately connected to your mindstream, will be embraced by your loving-kindness and that this seed will naturally grow within him, protecting him in dark times and leading him towards peace.
In times like this it is so important to perfect loving-kindness. That means extending it not just to your son, but also to your ex and his family. I recommend doing loving-kindness meditation as this will help to ease your mind, allowing you to be better able to support your son. With respect to your ex and his family, people are like mirrors and will, on a deep level, reflect what you present to them... So present love to everyone involved.
You had a child with your ex, so obviously you two had things in common and were in union at one point in time. It may be very difficult, but it is important to recognize that just as you found Dharma because you seek happiness, so too does he seek happiness. He may not know where to find happiness, so with your heart and mind point the way. Embrace the spiritual connection you have with him as a result of your former union and child as an opportunity to share Buddha's love and wisdom with someone who is deeply troubled. This good will helps to ensure that beneficial results come for you and your son, too.
Sorry for rambling.
My thoughts and prayers are with you and your family, as well as your ex and his family, that all may learn from these experiences and ultimately find their way to Dharma, peace, lasting happiness and the experience of unconditional love, that no harm may come to any involved and that their process of growth may be gentle but firm, and timely.