I have the familial AD gene in my family too and didn't find out until my mum became ill. Her father who had it walked out when she was little so they didn't know that he developed AD at a young age as well as many of his siblings. As a person 'at risk' I would want to know of that risk. If you contact the Rare Dementia Support they will be able to talk to you about it. From various speakers that have been at the Familial AD support group it seems that being open and honest and giving age appropriate information is the best way forward.
I have chosen not to be tested and your son may choose not to find out if he has the gene mutation and that's okay, it has to be his choice and he can change his mind at any time. There is a little Facebook group for people that are affected by Familial Alzheimer's Disease (worldwide) that you or he might like to join too. Lots on the group, including myself, are taking part in research and drugs trials which has really helped me to focus on something positive.
My daughter is 16 and she has known for about 5 years about my risk of getting it. She also knows that if I get it, she could too. We talk openly and honestly and she knows that she can ask me anything and I will give her an honest answer. I would much rather she came to me for information than to start googling and frighten herself and keep it all to herself. More people at risk choose not to get tested than get tested at the moment. I'm sure that would change if there were a disease modifying drug.
My daughter sees me happily going off to London each month to take part in the drug trial, she sees me crying when I've had a post lumbar puncture headache, she sees me doing media stuff about Alzheimer's (always with her approval) and she sees me sobbing when I'm sad that other members of my family are becoming symptomatic. I want her to see all of this to show her that it's okay to be happy/sad/angry at various times. She mostly sees me getting on with normal life which is mostly what she is doing too. I'm pleased that I have managed to make something very sad and upsetting normal and okay to talk about and have feelings about.
I wish you luck with whichever decision you make and feel free to message me if you want to chat.
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