Absolutely agree with sushigex.
I've had several geckos do that to me. I always put them back because I do not believe it has anything to do with aggression or submission in crested geckos (other gecko behavior is not the same as crested behavior since most crested geckos in the wild do not have tails to communicate with, so I seriously doubt they would use it as such a form of communication). I believe it's more of a defensive mechanism, he may see you as a predator and is attempting to get your attention on his tail so you grab onto his tail so he can flee.
When it waves it's tail I would put it away until it's calmed down, they try again. Hopefully the gecko will become less shy and more comfortable around you as he no longer perceives you as a threat. Has their been a change in his surroundings lately or anything new happened that could cause him to become more cautious?