Yeah I wasn't sure about the leash, but he is on a leash at the store and we passed a few dogs walking around and he didn't mind them, so I'm thinking it has something to do with the way my other dog thinks i'm his family and my puppy doesn't think he's my family. either that or relationships between my dad and I and our dogs.
Don't let them practice this aggression or else you won't be able to ever get them together. Did you introduce them somewhere neutral, like a park? Try walking them together….like parallel to each other. Have some treats to reward behavior that is calm (if either one is calm enough to eat).
Six months is pretty much like an adult, and IME it is hard to introduce adult basenjis to new adult dogs. Just keep calm, and don't try to force them together.
If you have a neutral off lead, safe space...like a friend's yard, or a tennis court, or training center or something, you could try to let them meet just dragging leads, but that is a little risky unless you really know what you are doing. If you are anxious, that can make the dogs anxious....if you intervene too early or too late you can mess it up. If they already have it in their heads they don't like each other, it might be too late.