Hi
Has your cat always done this or has this behaviour started more recently?
There are different factors that affect how a cat urinates. Firstly there is litter preference, have you changed the litter recently? If so change it back. Has your cat been frightened at all whilst going within the box and thus feels more secure if perched on the edge so he can run away quickly. Cats are vulnerable when urinating as they can’t defend themselves quickly. Other cats/dogs/small children may take advantage of this and pounce. Thus they perch to gain a better view of danger. Or it may be simply that he has begun to associate perching with going to the loo and this quickly becomes a conditioned response. Try using a box with a hood or a box with very low sides and make sure the box is tucked away in a corner so your cat only has to worry about what may be coming in one direction.
Rosie Barclay.