As everyone's circumstances, family history, geographical proximity to GC and finances are different, there isn't really a one size fits all!
Agree with others, adult grandchildren/nieces/nephews don't really need presents unless you wish to continue giving them and your finances allow this.
If he is your only GC and you can afford it, I would personally consider continuing to send a small present/money to him and review the situation if he settles down with a partner and/or has kids of his own.
Maybe continue with a small gift to your GS's half-brother anyway if you can until he is around 18 - seems only fair to him and your DIL to do so if possible.
Your DIL sounds lovely - is there any way you could visit for a long weekend and encourage GS to be there as well or, alternatively, invite them to yours? If not, maybe you could invite your GS to stay even for one night (maybe along with a friend/girlfriend) - just say you would love to see him sometime and he is welcome to bring someone with him.
You might also feel the benefit of writing texts/letters/ FB/WhatsApp messages to your GS and DiL telling them how things are with you at random times outwith birthdays and Christmas. Even if you don't get many replies, you may feel more in touch with them that way.
We have 8 grandchildren (all under 18), whom we are lucky enough to be able to see regularly. Adult nieces and our own siblings no longer get presents. Our AC and their spouses get birthday presents and, at Christmas, get restaurant vouchers (using supermarket points boost) along with a promise to baby-sit , so that we can concentrate on presents for the GC. We tell our AC not to buy us Christmas presents, so they usually just bring a bottle of wine or home-made goodies when they come to us around Christmas.