You have at least two factors working against you:
First, the mother cat is driving away her offspring, that’s natural behavior (as cruel as it seems to humans who have a different perspective on family). If your home and it’s surroundings are “her” territory, he may be permitted to pass through, e.g. to feed, but depending on her personality she may not tolerate him “loitering” otherwise. Remember, she was here first... Feral females tend to get along in female groups, males are much more solitary.
Second - and this meshes with the first point - he is now mature enough to find his own territory, especially if he’s not neutered. In that case his natural sex drive will make him wander around in search of females in heat. He may come back regularly, but it’s common for intact toms to “disappear” occasionally, even for days, or stay away one day.
If you want to keep him as a pet “at home”, your first step should be a trip to the vet to have him neutered. (If not done already.) This will not only prevent future stray cat babies (so the responsible thing to do for all cats that can go outside), but also diminish his roaming drive and reduce his stress level overall.
The second step is to keep him indoors at least for a while. Your home must become “his”, and that is best done if he can’t choose to go outside at will. Note that if you go that route, you will have to keep him busy and happy, so you absolutely must find the time to play with him, challenge him mentally and perhaps even teach him tricks on top of just feeding and petting him. Otherwise it’s not a home, just a prison. You have an energetic youngster at your hands, that’s work.
If your home has become “his home”, you can consider letting him outside again. Hopefully it’s attractive enough to keep him “at home” enough for your liking.
One extra remark: If the mother’s territory is still around your home, you need to pay attention to her behavior - there’s a good chance that she’s “discouraging” him from returning home. You may have to intervene in that case.