tough spot, but one that needs to be solved as your son's life is at risk.
is there somone on his team who's parent is your friend? if so, you could confide in them and ask them to drive your son home, with their child also in the car. that would probably not arowse suspicion from your AH...
or, you could temporarily drop out of your alanon, simply explaining you're not going anymore then you can pick up DS...you could attend alanon on-line or get private counseling on a different day...
or, you could move out with son and file for a divorce...