posting a photo would let us give you some idea, even better if you can get one of him with his tail out showing off for the mirror (flaring).
Crowntails have longer rays in their fins, that extend past the webbing.
Plakats have short tails, as mentioned, and can be any of the other types as well.
Veiltails are the type most commonly seen at pet shops, with tails that hang/trail behind them like a veil, and don't fan out.
Deltatails have tails that spread out like a fan, superdeltas spread really wide, almost up to 180 degrees, and halfmoons spread to 180 degrees wide, so you can draw a straight line between the top and bottom edge of their tails.
<-- halfmoon
figure out which one sounds most like your boy, then take a look here http://bettysplendens.com/articles/catview.imp?catid=857 for pics to confirm