I owned a daycare for 11 years and worked for someone else in their daycare for about a year. Number one is pick your schedule and stick to it. Things change and kids are able to adjust but from the start, put him in the daycare the same days and times every week. Later on, if you have a half day off and pick him up early, it's a big treat. If you put him in half days and then switch to full days, it will be harder for him to adjust.
Don't show your child that you are stressed over the decision or emotional when leaving him there. Go in ahead of time, take him with you. Meet his teacher, sit in her class with him and let him observe for an hour during play time or circle time. You are responsible for showing him how much fun the other kids are having. Encourage him to participate with you in the room. Then, when you leave him there and go to work, let the teachers do their job. If you hover, console, worry, get teary, you make your son resistant to the change and I promise you, through her smile, you will be driving the teacher crazy :-). She is trained to do her job, to know how to comfort your child and engage him in what is going on in his classroom that day. If you will bring your child in, help him get his jacket off and lunchbox put away, find a toy and a friend, then smile, kiss him, say goodbye and leave, the teacher will step right in behind you and take over his care from that moment. She'll know what to do.
He may struggle with the adjustment going from a one on one situation to a group setting, but a good daycare with a trained staff will have the tools to help him with that transition. Don't panic if he seems out of sorts for several days when he realizes you are going to drop him off. He may struggle for a few days, but it always gets better. The transition will probably be harder on you than it will on him.
Being a good mom is sometimes tough, but our kids are tough too and he will be fine. Good luck!