View this as a wonderful blessing! You have found such a wonderful daycare provider that your son loves his time there - he is safe, well cared for, and has even made a best friend!
I know, it is hard to think that your son isn't anxiously awaiting your return in the evening - but think of it this way. How would you feel if your son were standing at the window watching for your return? His little face sad, tears in his eyes?
I've taken my kids to both kinds of daycares. My older daughter went to a wonderful daycare where she loved getting there in the morning and hated to leave. Last year with my younger daughter we had the unfortunate experience of a daycare where she did not want to go in the morning, and was waiting at the window for our return at night. Believe me, you want the kind of daycare where your child loves to go and hates to leave!!
That being said, your best course of action is to arrive and tell your son, "Ok! Five minutes until we leave!" And then after you talk to your daycare lady and see how his day went, you tell him, "Ok! Two minutes and then we leave!" and then when it's time to go - just go. Pick him up if you have to, but go. Don't bargain with him. I've found that with a few minutes warning, my kids are much more likely to go along with the plan, without a fuss. They just have to have the warning of what's going to happen so they can adjust their mindset to it, and then know that whatever you said was going to happen, will really happen.
Good luck! Hang in there!