You mentioned you live next to this 'couple'. Maybe try the wife factor. Explain to his wife that having the vehicle parked so close blocks the view of on-coming traffic and is dangerous, making your wife feel uncomfortable and puts your family at risk. She probably has more pull with him than you do. And is probably a little less hot under the collar about your Dad's parking job.
If she snubs you too, then I'd go on to explain, as previously mentioned, that while city streets are public property, and her husband does have a right to park there, he has to respect the 5 m rule (maybe at this point offer to put up a marker if it would make it easier for him because of the size of the vehicle), and you would hate to have to involve the city to make it a safe place for your family, since anytime you have to involve the city it results in fines, which wouldn't really make for good neighbour relations, now would it?
Good Luck with whatever path you choose.