The majority of settlements between separating couples are simply by mutual agreement. (i.e. there's no court ruling and the Child Maintenance Service isn't involved). So if your son simply agrees to pay £x per month toward's the care and upbringing of his child (and his ex is agreeable to it) there's absolutely no reason as to why he would need to tell his ex about any extra money that he receives.
However if no mutual agreement can be reached, and the Child Maintenance Service is involved, then your son will be obliged to advise the CMS of any changes to his circumstances. Similarly, if a court has been asked to rule upon the division of the couple's assets it might be possible for his ex to seek a re-assessment if she becomes aware that he's come into a large amount of money.
So, if your son wants to hang on to his inheritance, he should definitely try to seek a voluntary arrangement with his ex.