The legal situation regarding child custody can vary depending on the country and jurisdiction you are in. It's crucial to consult with a family law attorney who is familiar with the laws in your specific area to get accurate advice for your situation.
In many places, when there is no custody order in place, both parents typically have equal rights to their children. If your son is the biological father of the children, he likely has a right to be involved in their lives and make decisions regarding their well-being.
However, if the mother of the children has passed away, it might complicate matters if her current husband is seeking custody of the children. In some jurisdictions, a stepparent may be able to pursue custody or visitation rights under certain circumstances, particularly if they have acted as a parent figure for the children during the marriage.
In any case, it's essential for your son to take immediate legal action to establish his parental rights and seek custody of his children if that's what he desires. This may involve petitioning the court for custody or visitation rights. If he is concerned about the children's immediate safety or well-being, he should seek legal advice promptly.
Remember, family law matters can be sensitive and complex, so it's crucial to seek advice from a qualified attorney who can guide your son through the legal process and help protect his rights as a parent.