Just spent two and a half years and $16K on custody case of my grandson--my son eventually won 50/50 physical and legal custody.
DOCUMENT DOCUMENT DOCUMENT!!
DB should keep a journal---every day--who did what for the child--meals, take to school, homework, etc. When he calls his DS, when she calls. Tell him to get as much time as possible with the child. Make notes after each and every conversation with the mother. If 10--the child can keep a journal too.
Check the laws in your state--where we live is referred to as a "one party" state. Only one party needs to be aware a conversation is being taped--that would be the person doing the taping. Radio Shack will become your best friend. They have devices that can record from both your home phone and cell phone. They also have printing caller id machines that will record all incoming and outgoing phone numbers, dates and time. This will help verify the number of times DS calls his DS and when she calls him, etc.
Make sure Db stays active in DS's school and outside activities. If DS is enrolled in sports/camps./etc all the better--esp if mother doesn't get involved and child "misses" he is with mother. Have DB obtain school records---does his son's attendance suffer when he is with mother? Does he miss more class work? Have Db get DS's medical records--who has taken him to the dr--mother or father. Is he up tot date on visits and immunizations? Dentist? Father should also voluntarily take a parenting class--most courts offer a divorcing parent seminar.
DOCUMENT DOCUMENT DOCUMENT
While most states have best interests of the child in the intent of the law--there is still a huge bias in favor of mothers. There are lots of websites that have great info:
http://www.deltabravo.net/custody/
word on guardian ad litems: be CAREFUL!!!!!! In some states/counties these are really just lay people who get a few hours of training and then are "qualified" to make life altering decisions on "behalf" of the children. My son did not have one.
REMEMBER: There is still a HUGE bias AGAINST fathers in the court system. He will have to fight doubly hard, have concrete evidence (journal, pictures, recordings, etc) and invest a lot of time and $$$$ in this fight.