Avoid his path and allow him to be his sister's problem.

[ Which is at least a little bit true. Viserys is a bit like a mad dog, or a mad dragon in this case. He can be set loose to create loads of chaos, but there's not really any controlling him. ]

I do not believe him to be a true threat, at the least. He hasn't the wits of his sister nor the strength of his brother. And if he has some ungodly ability given to him by the porter at his disposal, well, I imagine the world would have been leveled by now. As it has not been, it's safe to say he should not be too much of a problem.