Hi Judy,
Sorry to hear about your dilemma.
That's the problem with this brutal disease - no one can give any definitive answers because no one really knows.
I don't know if you will be able to find any resources that can answer your question whether or not you can do anything to stave off the onset of a familial form of ALS.
That's a tough call whether to advise your son now - as there is still no certainty that your son will get it.
In a lot of ways "ignorance is bliss" when it comes to ALS. For several months I just thought I had carpel tunnel syndrome - a minor inconveniance. Then after some tests - bang! - my whole life turned upside down after I was told I have ALS. Even though not much had changed from practical point of view - I felt like the whole world had been pulled out from underneath me and that I was free falling.
Positive attitude is everything when dealing with ALS. Although everyone and their third cousin has a theory about how we should be treating ALS - no one can really say for sure if it makes any difference. So sometimes I wonder if I would have been better off not knowing for as long as possible. Although I suppose I would have been a little freaked out wondering what was going on with my body.
As for the protein drinks - I believe that they are all quite high in MSG listed by a variety of names (eg. soy protein, hydrolized proteins).
See this link (courtesy of Jerry Story):
http://www.truthinlabeling.org/hiddensources.html
Although there is no scientific evidence directly linking MSG with the onset of ALS - MSG and other glutamates seem to have some role to play in the disease and it appears that they are best avoided by PALS.
If your son is health conscious you may suggest he avoid these protein drinks due to their MSG content for general health reasons without going into ALS.
You might also want to suggest he avoid processed foods and foods subject to a lot of pesticides (apples, grapes) - and try more organics where possible.
Will this make any difference? Who knows - but how can you lose by eating better.
Good Luck!
Richard