I am really sorry for your loss, I can't even begin to imagine what you must be going thro. I didn't answer earlier because I was hoping someone knowledgeable would come along instead!
The Dua I know is this one;
Inna lillahi wa inna ilaihi raji'oon. Allahumma'jurni fi musibati wa akhlif li khairam-minha
Verily unto Allah we belong, and unto Him shall we return. O Allah! Reward me in my calamity, and grant me from it something better than that.
Umm Salama used to recite it when her husband passed away. They had both loved each other a great deal, when she remembered him, Umm Salama used to recite this dua, and even so she would wonder who would be a better husband then her husband had been to her. She later married the beloved Prophet (saw).
Also keep making dua for your son, a parents supplication for their child is very potent, may Allah ease your pain, may you be reunited with your son in Jananh tul Firdows.