I am often amazed by the way people are so concentrated on the actual house they are buying without considering things such as access, neighbours, and possible change of outlook. Many people choose a house for the lovely view, without of course owning that view, and not checking on the likelihood of a big change such as more houses being built opposite or whatever. As I have moved quite a lot, I do tend to be careful about the little bits and pieces and check with the solicitor.
I think that checking the law with the solicitor is the first thing to do, as you will then understand the legal situation, but it would be much better to think of a way to
a) understand the present situation, such as checking with other neighbours how things have worked in the past, and deciding what is the way to go ahead without alienating your neighbours in general and them in particular.
b) how much all this is going to cost you compared with simply putting up a blind or curtain , or simply learning to ignore it and not letting it become a major issue
c) recognising that you may have moved there without considering what a terrace house living entails, and deciding what is really important to you. That if this becomes an issue that leads on to further upsets, that at some time in the future if you wanted to sell, this dispute would have to be mentioned.
So I really think checking with the solicitor is the first port of call and then it may be worth your son writing down the positives and negatives of the situation, now and in the foreseen future, and deciding if he can make a way of living with the situation, or improving it in some way, or at the worst, accepting for him, he really cannot tolerate or deal with this successfully and maybe think about moving. That may sound drastic, but if it is going to be a constant irritation and a point of possible disputes ongoing perhaps it might be the best option, and maybe a terrace house is not for him.
Generally I do see some problems likely to arise especially as many council houses were sold off during Maggie Thatchers era , which are now privately owned. However when they were built there were things like shared stop taps or water connections in the street, and I would suggest that everyone checks up on such things, as if you suddenly have a leak , you might find that switching the water off in the road also switches a neighbours water off that you may not realize.
Whenever I have moved (19 times as an adult both here and abroad) I have always found it worth doing several things, which some people think is excessive, but I have thought worthwhile and I pass them on . We actually generally give very little time to really thinking and checking out the nonvisible but nevertheless important things about a possible home in a new area. so
1. I look at a map and supposing I plan to live on the outskirts of a large town or city, look at which area has two possible roads in and out of the city. That way you look at a pie slice to give you a good choice of area, so that in the future if a major road is shut or has roadworks you have a second way to get home, especially avoiding areas that say have a bridge over a river where it is the only way across for miles.
2. I get the local paper of the new place sent to me for a few weeks. Reading this not only gives you a general idea of what is going on, good and bad, but also if you keep seeing X street being mentioned for fights, pub bans etc you can make a note that that is a street that you would not want to live in. Of course you can also see some streets that are ones you would like to live in, so in my case when I see open gardens in an area, as a keen gardener that would be somewhere I would want to look at.
3. Then when I am pretty sure that I want to make this move I go a bit further and deliberately go to stay in a b/b preferably during autumn or winter or not in a holiday time. This will show you what the town is like on normal every day times, so you get more of a true view . Staying in a b/b again is useful as the owners are local and can give you much valuable information, and also if you find them very good, I usually deliberately stay b/b for the first night when you move and sometimes a second night. That way you can work at your own pace and do not have to do the beds etc
I have thought all the extra effort has been worth it and so far, touch wood, have been happy with where I have ended up. Good neighbours are worth their weight in gold, and whether you become friends or just neighbours enough to take in a parcel when necessary, I find them an important part of my life. So hope your son decides what is important for him and is able to find a way to deal with things.