I would begin with putting up an ancestor altar. There could be a lot more to the relationship, but only a divination would tell me. As general advice, a simple ancestor altar would allow the two of them to interact in a managable and controlled way.
Set out a simple white cloth, or doilie, and put a picture of his mother on the cloth. Take a white candle, and some offering of drink (whiskey, coffee, tea, or even water is fine).
Light the candle, and call her name, ask her to join you in love and kindness, so that she may speak to her son.
Ask him to simply listen to the sound of the candle flame, he may hear nothing, or he may feel her.
If after three attempts (no more then one a day) to make contact fails, use a railroad spike, or a hammer and knock three times on the surface that the altar is on.
Generally, i make ancestor altars this way:
I do not put this altar in a bedroom (although some people, like Miss cat, do so and find no problem with it).
I do not use a picture with living people (although some folks do, if they want the dead to protect the living).
I offer incense, candle or drink once a week.
Additionally, I have found that putting a doll, a nice one, on the space helps with the connection. Let them both speak thru this medium. Some people use a statue for this purpose.
This forum thread is on ancestror work:
Ancestors, The Dead, and the Graveyard Questions and Answers
ancestors,-the-dead,-and-the-graveyard- ... 15987.html
Leah of Earth and Spirit
(Formally Lara of Black Cat Root Shack)