You probably have many responses by now, but I just wanted to suggest that you coordinate with the day care teacher for an exit strategy that allows you to say goodbye to your son before leaving.
His behavior is very typical at that age, and be prepared--it could last a while. But I've always believed (and the staff at the day care center I used agreed) that it's best to say goodbye with hugs and kisses, telling him you'll be back later to pick him up, and then leave quickly--even if he's crying and clinging. That's where the teacher should be ready to peel him off you and distract him with a great toy or activity while you exit.
It's heartbreaking to walk away while your child is crying, but he'll soon stop after you leave. Telling him allows him to develop trust that even though you have to leave, yes, you (or daddy) will be back for him later. Not telling him will confuse him and leave him without any reassurances (which he may not understand now but will come to understand in due time).
I remember those days so well (my little guy is 7 now)! I often found myself in tears as I drove to work chock full of mommy guilt. It was tough, but he'll outgrow it and won't even remember it. I just wish I could say the same for us mommies! :)
Best wishes to you!