Theres the Good Schools Guide who claim that their comments are made after they've visited a school and interviewed lots of teachers pupils parents etc but DS's prep was unrecognisable from their review! They're unlikely to write anything negative and I find some of their comment rather gushing. Tattler writes a slightly amusing similar and accurateish thing every year, But the schools know they're coming and will definitely be painting the roses, polishing the children and hiring goldfish for the visit!
You could post on here, there are quite a few of us with children at a variety of DS's and DD's a boarding school but some comments will be over fullsome their praise or contradictory and quite negative you will also get suggestions from people who don't have DC's there but have heard on the grapevine that X is great.
A boarding school's ethos must suit your DS and you as a family much more than a day school. The great adage one mans meet is another mans poison very much applies to boarding schools, I can think of a number of schools I wouldn't touch with a barge pole that many rave about and I know people have looked at DS's school and don't like it. You will also find current parents who have very different experiences, it's very dependent on the staff you and your DS meet and are involved with, if you and your DS are in step with the general ethos of the school and if it's living up to your individual expectations.
My advise is start of with a list of must haves, for example coed/SS, how selective this will be influenced by your DS's predicted grades, location; rural/market town/city, and most importantly location in terms of distance from home, there are many seasoned boarding school parents on here we would advise you 1 1/2 hours (one way ) max travelling is what you should be aiming for, of course if you live in the Scottish Highlands then you might have to reconsider this. Also equally important what sort of boarding do you want? Full, that means staying in school 7 days a week, perhaps being allowed out on Sunday and on set weekends, weekly, coming home on Saturday going back on Sunday night/Mobday morning or flexi boarding a few days a week. Full boarding only schools (or with just a handful of day pupils) are very much in the minority, most are weekly boarding with day pupils, it's slightly less important at 6th form level as 6th formers are more likely to stay at weekends even in predominately weekly boarding schools but IMO if you want full boarding choose a full boarding school it will have a different feel to it than a weekly boarding school with some full boarders. If you DS is a golf nut or a the world croquet champion do make sure they can offer his sport, however mainstream it might be don't just assume they'll do it. I don't hide my loathing of meaningless ritual and outdated ridiculous uniform, many boarding schools will have both of these in spades decide if you or your DS want this. If he's arty look at art depts ditto music, or if he wants to do an uncommon language is it offered? Do you want a big name with matching fees or a relatively unknown with possibly cheaper fees?
Look at your DS, boarding IME is a positive life changing experience but only if it work, those that do best are robust easy going types, it's not always easy to join in the 6th form because friendship will often have been established for three years, and the next two will go very quickly is your friendly and outgoing? If I was in your situation I think I might seriously look at schools with a significant intake in its 6th form or perhaps one that has separate boarding houses for 6th formers, this doesn't have to be written I stone but definitely something to consider. Don't look at loads you'll get confused, look at three or four, try and visit each school on a number of times and if possible not just on a open day/morning, watch the pupils and staff, when they're going about their every day lives they should look relaxed (your in their home) but purposeful, relationships between staff and pupils are more informal, and when you see pupils together in groups they're likely to be more cohesive and pupils will often sit physically closer to each other these are good signs. Ask difficult questions, if you here the same answers "it's the best school in the world with the best facilities, opportunities staff etc" then this is just someone telling you what they think you want to hear. No where is perfect it all about it working for your DS. Do think about ethos DS's school is very liberal with few rules I like this others might not, it's hands off parenting I also like this, a friends with a DS at another boarding school gets a brief report every three weeks she loves it, it would take me four weeks to work out how to log into the schools system to access it! Also although not majorly important how sociable do you want your school to be? DS2's school thankfully doesnt organise endless social events for parents but others do.
Others on here will come up with other things to consider.
What I'm trying to say amongst other things is that choosing a boarding schools a very personal thing no website or school advise organisation can really help you because we all see things differently, you can get general advise e.g. thats definitely a super selective full boarding school who don't do rugby but the sort of things that will make it work for your DS and you can only be found by visiting a school a few times observing it got yourself and asking the right questions.