Cichild behaviour is as variable as their colouration. You could have a female, which will turn a little aggressive but nowhere near what a male might. Or you might have one of those rare RD's that just isn't aggressive. It does happen.
6" in 6 months is a really good growth rate, but it might mean that his personality will take a little longer to catch up with his size. 6 months is still pretty young, and aggression tends to come with sexual maturity. As Gerry said, 9 months to a year old is closer to that maturity, so it may just be a case of giving him a little extra time for his mind to catch up with his body.