How annoying! I think it sounds like your neighbour is not using the badge for the purpose it was intended e.g. its not for 'reserving' a space for his son whilst he parks elsewhere. Def sounds like misuse of the disabled parking badge on his part. If he regularly parks elsewhere then he has no need of the damn badge. Can you have a word with him about it face to face, doesn't need to be confrontational - just say what you have said here, that you have noticed he is not using it and that by parking elsewhere e.g. not in the bay, that this has an impact on the rest of the street. Knock on effect is that there is less space for others, if he used the bay that would free up some room etc. I wouldn't park in the disabled bay.