It may be difficult to find your son. 

You need to recite the Golden Light Sutra three times. This will help you in general, to purify negative karma created since beginningless rebirths, to accumulate extensive merit and to actualize the path to enlightenment, so it will be good for you. Also, by reciting the sutra you may come to know the story of what happened.

The Nechung Protector puja also needs to be done. Ask the Kopan lama gyupas to do this.