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!        

I can relate to this...

I was diagnosed at age 43 and so many things began to make sense. When my dad still had his shop in town we saw each other every week or two normally. I'd meet him for lunch usually because his house was about two hours away. He was preparing for retirement at the time and after he retired and moved full-time to his house with my step-mom, I only see him about twice a year and only talk to him about once every few months. My dad is JUST like me and after my diagnosis, I knew he was as ADD as me. Even though we don't talk regularly, we are as close as ever when we do talk. The thing is, neither one of us like talking on the phone. I'd love to see him more, but my DW and kids make it real hard to make dad time with our busy lives. 

I know it Should be different for your Ex, because your son needs his father. If your Ex is untreated for ADD, he may simply be operating on the "No news is Good news" strategy. It's not right, but this could be part of the disconnect. I've always said "The phone works Both directions". I hope your Ex improves for your son's sake.