Can any ADD fathers please try to help me understand

I have posted on this topic before but wanted to repost to see if anyone else had anything to say on this.  My husband works in another state than me and our 21 yr old son.  My son has had a lot of difficulty with ADD and depression and still struggles today.  My husband seems to never call him to see how he is doing.  Months can go by and sometimes after that all he gets is a text.  My husband calls others once in awhile in his family who he isn't close to so why doesn't it dawn on him to call his son.  I cannot understand it!!  It still bothers me so much and makes me wonder who the heck I was married to for over 25 yrs.  I know he would say he loves our son but how can you not once in awhile place a call to your child no matter how old they are.  Is it guilt over leaving, does he not know what to say, is he just waiting for his son to call him so he doesn't have to deal with it himself??  If there is anyone who has done this and knows why, i would love to hear from you.  Thank you!