Tell him to check out the Holiday Park Action Group. They are on Facebook and have a website.
The short answer is don't even think about it. There is very little regulation on residential sites and none at all on Holiday parks. Many holiday park salesmen will tell you that you can live on their parks, but you cannot do so legally, and they can (and do) evict you for next to nothing. Homes are expensive and lose value overnight, and you can't take them elsewhere. Rents can go up exponentially, and you probably have to sell back to the park and get any work done through them, paying them a huge commission (decking, wifi etc). You own the caravan/chalet, but the land belongs to the park owners, who can do what they like.
Seriously - don't do it, and that applies absolutely to holiday parks, too.