Suppose someone is raised without much authentic Judaism but with some. Perhaps his family has a monthly Friday night dinner with kidush; perhaps he wears t'filin for a few days when he turns 13.
Then, as an adult, he finds religion. Not having a family m'sora/minhag, he can pick whatever minhag he wants to follow, so long as it is an accepted minhag of the place he lives. But does he need to follow his family's minhag where it exists? For example, suppose it happens that the family poured water over their hands before kidush (which is the minhag of the Yekkes). Does he need to do that?
Assume that he has no idea (and no way of finding out) whether the minhag in question started with his own father or was passed through the generations to him.
I'm not asking about talis, which is a more complicated issue. My example of washing before kidush was meant to be an uncomplicated one; if it, too, is complicated, then please ignore the example and address only the question, whether such a person must keep a known minhag of his family.