It’s hard to get some cats to get along. It could be a personality clash, or maybe he’s acting out because he’s bored, stressed or is worried the younger cat is trying to take the alpha role. Usually, by the six-month point, you see some improvement in their relationship.
You can try the Re-Introduction Technique even though it’s been six months. Keeping them separate for a while might also be a good idea since the one scratched the other.
You also might want to put a Feliway Classic Plug-In Diffuser in the rooms your cats spend the most time together. This can help make them feel calmer, and might help with the older one’s aggression.
Once they’re together, try to encourage them to play because that seems to help with bonding. Don’t use catnip because it can make cats more aggressive.
If those things don’t help, you can ask your vet about medication to help calm the older cat until he’s not continually hissing and going after the younger one.
I hope it works out between your cats and thanks so much for your question!