Until both of your rabbits have been spayed and neutered, they should be kept apart. You can keep their pens near each other, but it’s best to keep the rabbits themselves apart for now. The first step in bonding rabbits is for both of them to be spayed and neutered and to have the recovery time from that, which is 2 to 4 weeks. Neither of your rabbits is old enough for surgery, so you will have to wait for them to be together for a few months. Unaltered rabbits can and probably will fight as they reach sexual maturity, and can severely injure each other. They can be sexually mature as young as 3 months old.
Here is one good article on bonding: http://rabbit.org/faq-bonding-multiple-rabbits/
It sounds like the new bunny is just being playful, not aggressive. At 6 weeks old, (which is too young to be taken from his mother), he likely wouldn’t have learned aggressive behavior yet, unless his siblings bullied him somehow. He’s excited about being in a new place and seeing a new rabbit, so the nipping is probably just a playful behavior that he learned from his litter mates.