I wouldn't worry about it too much. He's discovered how easy it is to urinate in places other than the toilet and it's a new game to him. Don't make too much fuss about it and he'll probably tire of it.

You might even find it useful in the future - the next time he needs to go while you are out and there are no toilets nearby he will have no problem urinating into a drain or an empty drinks bottle. Some children would rather wet their pants than urinate into anything other than a toilet.