Hmmm - this sounds like a complex problem that may have been many years in the making. When you try to communicate with him, how do you do it? By phone, in person, by email?
Additionally, 19 is a difficult age, as I'm sure you are aware. He may just be going through a period of adjustment that has little to do with you. The late teens and early twenties are a period of experimentation & he may just be caught up in his own identity right now as well.
Okay - well it may just be a case of him attempting to individuate - to establish his own adult identity away from the parental unit. This is normal for someone his age and I don't think it should be viewed as something that is necessarily pathological unless he has somehow indicated that it is.
Calling him every day might be too much contact for him right now at this stage in his life. The late teens and early twenties are a fiercely independent time (for most boys, anyway) and the continued contact might just be too frequent for him. Have you considered a less direct form of communication like email or text message? This would allow him to respond on his own time table. Or, have you tried cutting down to several phone calls per week rather than every day?
Well, you can always say that in an email or a card. Luckily, by virtue of you being his parent, he probably already knows this. When we're his age we take for granted a lot of things (health, family, safety) - it's just part of growing up. Eventually he will come around to appreciating the depth of his familial relationships again - it just might be a few years. It sounds like you are very caring and open - he is lucky to have you. If you remain open to him whenever he asks for assistance (without pestering him too much!) he will come to you when he needs you. The bond between parent and son is like no other.
He'll come around.
Oh my goodness! That's a question for the ages, I'll tell you. If I could answer that one, I'd be a genius sought out by millions... I wish I could alleviate some of the stress there, but I have a feeling that every parent goes through this during this stage with their kids. The best strategy that I've seen is just to stay in contact on a regular (though not too regular) basis. For example, some parents have an agreement with their kids that they will touch base every Sunday afternoon. It gives the child some space, but it ensures that the parents know what's going on from week to week. Another thing you can do for yourself is to get together with other empty-nesters and talk to each other. It will probably provide some comfort without putting the burden on your son.
Finally, the biggest thing I would suggest is to do something nice for yourself. Do something pampering and congratulate yourself on being a good parent. Your son's distance is actually a sign that you've done a lot of things right!