If he runs and swishes his tail slowly it means he feels threatened by you. It's probably best not to bother the young ones too much by trying to handle them. They get used to you and tolerate being handled as they get older, but in my personal opinion I wouldn't handle/pick them up them too often then either. 
THE ONE YOUNG LEO I HAVE IS WEIRD EVERYTIME I TRY TO PICK HIM UP HE WILL EITHER RUN REAL QUICK INTO HIS HIDE OR HE`LL HIDE AND I`LL C HIS TAIL IN THE AIR SWAYING LIKE BACK AND FORTH WHAT DOES THIS MEAN? IS HE JUST NOW USED TO GETTING HANDLED YET WILL HE EVENTUALLY CALM DOWN?
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