What to do with my teen?

My 15 year old son chose to live with his father since last year and is rejecting me completely( I got re-married). The problem is that he is not very well looked after at his father's house. He smokes pot and is allowed to be out of the house until 10pm on weekdays and weekends are even worse! Whenever I ring him, he is 'out' with his mates and last night came back at 12pm!! I have contacted the social services and they are planning a meeting with his father to put some boundaries on him but the process is quite slow. As you can understand, my concerns for my son's safety and well being is growing everyday. Spoken to ex many times, futile. He seems to be a careless man whose aim is solely to get into me through our son. He is not taking my phone calls or replying to my emails expressing concern over my son. I feel powerless! Some friends have suggested taking a step back and not contacting my son for a week to let him miss me. I am finding it difficult to do that since every day the concern for what is he doing outside on the street is pulling me apart. The school have written a report saying his grades are falling but nothing seems to bother that father of his who is busy in his profession and planning for his wedding due in August this year. Any suggestions to solve this critical situation would be appreciated. My new husband is talking to my son sometimes on the ill effects of weed etc but is not in a situation to have a dial-ague when his father is clearly giving him the money, freedom and negligence to down that path. I want to help my child. Thanks for reading this post.