Authority For Visa For Stepson

[kiwijack]

My Thai wife, ( registered marriage ) of 3 years, has a 6 year old son from previous relationship,( never married ) but son registered under fathers name. A common enough situation. Wife left his father some 5 & a half years ago & has raised son exclusively, without any financial assistance or visit from natural father since. Current whereabouts of father not known though both mother (wife ) & the father know where the respective family homes are. Wife is fearful of losing son if she approaches the fathers parents re contact detail for father of her son. Totally loath to initiate any contact through fear also of "extortion" & other problems as he is/was a loser with gambling habit, a case of letting sleeping dogs lie. Problem is we want to get visa/ passport for son but require fathers consent or custody judgement. Has anyone been down this latter path? I would appreciate any advice re lawyers , processes etc to obtain legal custody. Or do we have to wait until he is 18 years old? Hoping someone can help. Thanks.