I haven't read the whole thread, so sorry if I am repeating what others have said. It's a tough decision to make as there is so much riding on it, both for your ds and for you (as you need to be sure you have got it right, especially if you are overseas). My ds1 has boarded since he was 9, and we have just been doing the rounds of senior schools.
One of the best bits of advice I got was to make sure the school we went for was a boarding school as opposed to a day school which took boarders, so that there were other boarders around at the weekends. Ds1 is at a boarding prep school, and there are several weekends a term where all the boys have to be in school. They can weekly/flexi board in their first 2 or 3 years, but have to either fully board or be day boys higher up the school. Most do full boarding to prepare for boarding at senior school.
When looking around at schools I went a lot on gut instinct. You (or in our case, our employers - dh is posted abroad, and I will be going next year) are paying shed loads of cash, so you should take for granted the academic excellence/sport/music whatever. For us it was really important to make sure the pastoral care was good, as ds has some medical issues. We were given tours of the school by some of the older boys at all the schools we looked at and I asked myself "would I like ds to turn out like this boy?". In several cases the answer was no, so we didn't choose those schools despite the fact that we knew people with children at those schools who were very happy.
I feel quite strongly that ds should attend a mixed school, so although we looked at one single-sex school (recommended by his current head as a possible good fit for ds1) the others we saw (and the one he has been accepted for) were all co-ed.
If you are overseas you will need to appoint a guardian for ds (e.g. if he is taken ill/school is closed down because of outbreak of illness), so it might be worth thinking about looking in areas where you have family/friends.
Good luck in your searching.