4organicbabes, I totally understand! DD is 17 months old too. We have someone come to our home to watch dd, but it is the same thing. Sometimes, when our babysitter comes, dd becomes clingy immediately and starts waving bye-bye to her. She knows that I usually leave when she comes. The only advice I can give you is that it does get better. Also, the more drama you create around the situation, the more stressed out the child becomes. I have literally stood outside the door and listened to dd, crying, only to stop 1 minute after the door closes and start laughing. My heart would be breaking, but she would be fine. That's the good part. Toddlers this age (1) have short attention spans (2) short memories (3) are easily distractable. Between these three things, the baby that was crying its heart out when mom left turns into a total social butterfly afterwards. After standing outside of our door for about a 2 week period, I am totally convinced of this factor.
The other thing is that if I acted sad and let dd see that I was disturbed, she would get even more clingy. Instead, I tried to act calm and happy and give her a kiss and hug and then leave. If I prolonged it too much, she would sense the weakness and get upset all over again. After about a month, however, dd kind of grew out of it. Now, she even waves bye-bye to me. OR, she runs to the door to give me a kiss goodby. That hurts my feelings, sometimes, but that's a whole separate thread......
Anyway, it sounds like you and your dh are doing everythign you can to make it easier for your ds. Make sure he has your undivided attention when you are at home and he will be fine. I promise. Good luck!