M.B.
Life is so, so not fair. Dads can ignore their children for years, decades even, and the children will search for them and love them just as if they'd grown up with them. ARGH!
But, if the Dad is truly a bad person, at a certain age a child will be able to recognize it. I think the best idea is to let the child visit the dad, (not have him as a enemy-cum-martyr), and make a deal that at a certain age the child can go live with him if they both agree. That's reasonable, doable, and helps the mother-child relationship. A good age for a son is maybe 12 or 13 (maybe make it a 'teen' thing).
A good thing about that age, is that by then the child is able to know wrong when he sees it, and make his own judgments. And, the good thing about an agreement is that it help the child be patient and not demand it at too young an age.
Oh, and ignore that post about not taking your child back if he wants. You can probably send him without giving up custody, and I personally would always leave a door open for my child. AS IF he can really make such a permanent decision at age 14!