This is normal. He's going to take a couple of weeks.
How you can make it easier on him.
Tell him, in the car, that you love him and will see him later. Then head in without talking about anything to do with how he's feeling or going to adjust or going to be okay, nothing. You'll be making him feel like it's not normal and he has to fix something.
Tell him bye when you hand him over to the teacher or he leaves your side to go into the classroom.
Turn around and walk away. Do NOT look back, do not hug him and say it will be okay, don't do anything or say anything, just walk away, all the way to the front door and out to your vehicle.
Also, you might know him better than us but by not taking him in every day you're giving him a mixed message. He knows you have to go back to work and that's great. It's not harmful to him in any way. But by having him home several days of the week it's confusing to him.
PLUS by picking him up at the end of nap time you are setting him up for a huge melt down as soon as you go to work and don't pick him up at this time. He's going to get up, be half asleep, and go get his stuff ready and sit and wait for you. When you don't come he's not going to want to go have snacks, he's waiting for you, then he's not going to want to go outside and play because he's waiting for you and something must have happened, he thinks you've forgotten him, so he's got to sit right there and wait so you don't forget him.
He's confused and really waking up now but you're not there and he can actually get frantic because this has never happened before.
Taking him for a shorter day is not good for him. He needs to be there the entire time you'll be working so he knows what to expect, when he might change rooms, who the other staff are because a LOT of child care places hire high school students to come in after school so their full time teachers can go home and the high school teachers come in fresh, they're young and like to play with the kids, and it's just a good thing.
The most important part of what I have to say to you.
Get him to full days for the whole week soon. He needs to be there the full time so he can really get used to it. What you're doing isn't going to do it all the way and he's going to be right back at the beginning of adjustment when you do take him for full days all over again.
Taking him in and turning around then leaving will make the adjustment go smoother and quicker but getting used to the schedule and going every single day full time will be the beginning of his adjusting.
What you're doing now isn't even starting the adjustment or transition.