yes ther is a connection.he has to know he doesnt have to suffer with depression but it is hard for many men to admit they need help. It is often seen as a sign of weakness when in fact the head isnt getting the right connections to the brain with dopamine and seritonin. see if he will look at these posts here and maybe we can help him to get help.keep telling him about the options he has and let him know because you love him that you are concerned.
I will keep you and him in my prayers.
depression can wreak havoc on the immune system. also, depression can give one flu like symptoms, fatigue, weakness, aches etc. let him know that there is no shame in getting treatment, quite the opposite-- one is courageous when they decide to face the illness.