If they've already been through puberty and gotten in the habit before hand,
they sometimes don't stop, at least not right away.  He also might be doing
this if you have any other pets, or he's feeling a need to mark his
territory to make himself comfortable with you.  I've not tried it, but a
lot of people have recommended Feliway for this sort of thing.  I don't know
what else to suggest, but hopefully someone else will have some other ideas.
Very kind of you to look after this guy and find him a good home!

Best of Luck,
Ann

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http://www.angelfire.com/ca/bewtifulfreak