3 Weeks in Istanbul

what are some of the more out-of-the-way places you have discovered in the city??

I would think the girlfriend and her family would have the answer to that question.

Any other thoughts about culture or customs he should know since he's staying with a Turkish family?

If he and his girlfriend are at the point he is visiting her family, ( for 3 weeks) I would be surprised if she has not told him about such things.

I wrote a comprehensive list for someone but of course the search function is so weak that I cannot find it. The short version is any place outside of the Old City/ Sultanahmet (i.e. tourist central), including the Asia side. Istanbul is HUGE, with layers and layers and a maze of different neighborhoods - from old and scrappy to modern high-rises and business districts. I would try to see as many as possible, including along the upper Bosphorus and the Golden Horn (including Eyup, where there is a very old conservative mosque). I think the best thing would be for the girlfriend to take him to her favorite places, there is nothing like a local showing you around a giant, complex city like Istanbul. Most areas are full of locals and have virtually no tourists. One of my favorite trips was a ferry down to the Princes Islands, which is a nice respite from the human traffic of Istanbul (I bet the islands are much more busy now than when I went).

His girlfriend has likely already filled him in on this but.... probably he'll be expected to take off his shoes when he enters a home, and wear socks or slippers (provided by host). Also, it's quite possible that they do not imbibe alcoholic beverages at all....

Thank you so much Agnes; this is helpful - we haven't heard of some of these so will add them to the list of things to possibly see.
For those of you saying that I don't need to ask since her family lives there, I would say this. I live in Los Angeles and don't know every single nook and cranny - I'm sure there are many places I have yet to discover that others may know. I believe that people who live in other cities as well maybe don't have a comprehensive knowledge of every single thing. I think it's fine to ask questions to find places that other people have enjoyed...but thanks for the lecture anyway.

Middle Eastern history covers over 4,000 years. He may have a particular period in which he is interested.
Gallipoli, a major WW1 military disaster for Britain/ victory for Ottoman.
Rhodes and area for archaeology
Ephesus for same
Troy for same
Cappidocia for amazing geology/geography/human settlement
And back in Istanbul, many days in the archeological museums
and wandering the neighborhoods.
IRT family culture, I hope he and she has figured out sleeping arrangements. There are many American families that would still be upset if their daughters' boyfriend was too friendly/intimate with her in their home. There is a reason that American humour has references to "shotgun weddings".

Turkey is a secular country. Many Turks drink alcohol. Many Turks are physically affectionate with each other before being married.

How exciting for your son! I’m sure he will have a wonderful time. One somewhat off the beaten path is the Pierre Loti café. It is high on a hill and has a great view. Lots of locals go there.

Ten years ago we spent 5 weeks traveling around Turkey and there is LOTS to see. However, fairly close to Istanbul is the island of Bozcaada which we loved. A little further but not hard to reach is Cappadocia which is full of interesting sights. What a great experience for him.