Will it continue for the rest of my life?

My ADHD husband and I are in the beginning stages of a divorce after  29 years.  We have a 15 year old son.  After two years of trying to get him and us to get some help he left.  The thing is, we still have a minor child.  We need to communicate and be able to work together.  It was very bitter for a while after he left. It has been almost 5 months.  These past two weeks I have seen a difference in him.  He has been calmer, kinder, helpful and cooperative.  He helped me out with a couple of issues I had with my car and we were able to talk about how Christmas would work.  Today I needed to speak with him regarding some things I was looking for in the garage, things I needed for house maintenance.  I call him. He doesn't pick up.  I text him.  He doesn't reply.  My son calls because he was helping me.  My husband gets impatient and yells at him. My son calls him later and he won't answer.  It is so confusing. One day he seems normal, calm and even kind.  The next day he is the total opposite.  Our son has only a little over two years then he will be an adult.  I know legally we won't have shared responsibility but I think of all of the events we will have to share in the future and dread the thought of having to deal with his unpredictable behavior.  Is this strictly ADHD or could there be something else going on?  My son today (without me saying anything) made the comment "What is dad, bipolar?" because he sees how his dad is so different from day to day.