I've used sandpaper on plastic and terracotta to file down sharp edges, so you could probably give that a try if you find anything. Just wash it off well so there's no residue.
Another option is to try to "tuck" away anything sharp, so he can't catch his fins on it.
And I bet you could coat things with aquarium-safe silicone to coat the sharp parts. I picked up a big tube of Loctite Clear Silicone Waterproof Sealant at Lowe's recently. It labeled "aquarium safe" and cost about $4.50, and it'll last me a really long time. (I'm using it to make all sorts of decorations now. I just made a little rock 'bench' for my new fish. It took him about two minutes to swim underneath.) If you do this, make sure you get the one that's marked "aquarium safe."
(Not sure about the gravel. My tanks are all bare-bottom. I don't have a filter, so I vacuum frequently with a turkey baster, and it's easier to do this when there's nothing on the bottom.)
Last edited by LittleBlueFishlets; 11-30-2012 at 10:00 PM.