I would be fairly honest at a level he can understand.. Mummy is going on an aeroplane for a long visit but will be home in .... sleeps. Maybe make a simple chart with box for each day for the fridge so he and Daddy can tick off each day till you are home. so the emphasis is on when you are back rather than you are gone IYKWIM.

Great idea!!! I'd do that too!!! Keep the explanation as simple as possible. Look after yourself as well. It's hard work when parents are unwell xxx