I think 6 mos is too young to start ignoring a baby's needs. They cry for a reason. They cry when they are hungry, wet, sick, or maybe just scared of being alone. I believe in parenting a child based on his or her needs. Some kids can sleep fine on their own, even at 2 or 3 mos (or so I've heard!). Some physically need to be next to mom or dad to feel safe enough to sleep.
It's wonderful that you have been cosleeping this long, though I don't know why you are wanting to push him out so soon. You didn't mention any problems. Sounds like sleeping next to mom and dad is great for him, why are you in a hurry to stop? Are you getting pressure from relatives? We've had grandparents say it's bad to let kids sleep with their parents, but I trust my own instincts and we are all doing fine. (One grandma now bugs us because our son won't sleep with her (like her other GKs do) when she visits!)
If the crib fits in your room, start there. Set it up next to your side of the bed, maybe next to the wall and push your bed up to it so it's a tight fit. Lower the bar or take it off and you have a sidecar bed. You can start sleeping together and after nursing push him over into his own area. Then he is close if he needs you. If he seems comfortable in his own space, you can try with the bar up (you'd probably want to do that for naps anyway) and eventually move the crib across the room and at some point into his brother's room. Then when he's older he'll get into the routine of going to bed like big brother.
Just be sensitive to your child's needs. If he cries, pick him up. That is not spoiling him. Spoiling is when you ignore something and it rots. Be a postive parent and portray LOVE to your children and they will receive the message and reciprocate.