Hi L. - I believe it is one of two things:
1. Stress associated with day care - and that is perfectly normal and may even be his reflection of your stress. Start associating day care with all good things....be all excited that he going to see his friends and the caregivers - make sure they make a fuss over him when he gets there; if its an emotional response he could get over it that way. Also, the excitement of being with the other kids, even if he loves it, can get his little system going. I bet it will calm down eventually. Another thing to do to help him like daycare is not to "grab and run" at the end of the day - take some time to explore the toys, friends and other things with him shows that day care is not a place to be running away from - its a great experience and something to look forward to!
2. Something he takes only to day care with him or something day care is feeding him; I am thinking water - does day care fill his water bottle or add water to something he is fed? Their water may be different from what he is used to. They may also wash his pacifier or bottle with some soap he is reacting to.
At his age, he should not be experiencing stress from going to day care - he does not really get separation anxiety till about 18 months or so, so he may be feeling your stress and that is what he is reacting to. Even if it kills you (believe me, I remember), keep smiling and keep him distracted when you head out the door - make a game out of waving bye bye and have a little routine for bye bye that makes him giggle. Remember that you placed him there because you believe it is safe and happy - and if you don't start looking for somewhere else!!
Finally, let me tell you what you are in for.....I thought those day care separation days were over till my daughter started COLLEGE this past Fall - and its brutal because by then the kids are SO ANXIOUS to get rid of the parents (they literally stand there and say "WHEN are you LEAVING??" - sniff, sniff) and the parents are SO SCARED....it's pretty funny! But, give it two or three weeks, and the kids are really happy to head home for a weekend or the holidays - they find out they miss you!! After a year (almost) my daughter has also traveled TO SPAIN (wait till you feel nervous for an entire 10 DAYS - that's how we get gray hairs!) and shown me that I did a pretty darn good job raising her.....and it feels good!!
Final advice.....all things pass....but they seem so difficult while you are going through them (this from the woman whose child BIT for a whole year - the other day care families thought something HAD to be going on at our house - and there wasn't!!!) just when you think you are through something there will be another challenge...trust your instincts....you DO know best....and, save for college, the time just goes by toooooo quickly to get it together later....even a tiny bit now will help A LOT later!