I wouldn't do a reward chart over something emotional like this. I'd try to support him, he is very little.
It's just the hump of goodbye and he recognises that. So I'd acknowledge that's hard for him, express that you know he's brave and can handle hard things, and see if there is anything that will help. Part of learning emotional regulation is having many many experiences where they go through the hard feeling/emotion, experience that and then that they are ok afterwards. Learning that hard feelings are survivable is important, not pushing them down and pretending they aren't there.
For example
Speak to the teacher and see if any other children struggle with this change in drop offs and what they do. For example they might be able to go in with a TA, or another child assigned a job to look after them, or be offered a job to do (maybe even looking after another child, or the class bear who feels a bit worried with all the noise when everyone comes in) first thing in the morning. The teacher might also be able to tell you if they have noticed anything else.
Do you walk to school, can you try to walk with someone else local so the DC can go in together?
Offer some kind of transitional object, such as a small item he can keep in a pocket to secretly touch and comfort him when he's not at home. Or you can also use imaginary objects here. There's a book about an invisible string iirc which talks about how people we love are connected to us and the string can stretch and stretch as far as you like but never breaks. My eldest also used to like it when I gave him some "spare kisses" to put in his pocket in case he had a hard moment of his day. We'd have our normal hug and kiss and then I'd blow 4-5 kisses which he'd pretend to catch and put in his pocket.
Social stories can be very helpful. This is a technique developed for autistic children but they can benefit any child finding a situation daunting. It's basically a short book using drawings or even better photos to show the process of something that they can sort of "walk through it" on their own to practice. Include pictures of the child and include any common scenarios like "Sometimes David feels sad when it's time to say goodbye. But when he gets into his classroom and sees his teacher and his friends, he feels ok." And if there's anything he can do eg he can talk to the teacher if he's worried about anything during the day.
If he tends to be fine once he's in the classroom, a technique that reminds him the sad hump is just a hump and he can get over it can help. For example, if he can count well, try getting him to count how many steps it is until he feels better, and tell you at the end of the day. You might find that he starts off with "today I was sad for thirty steps" and then it changes to "today it was only 25 steps!" If he's having a bad day, it might be "today I didn't even feel better for a hundred steps" and you could sympathise and say ugh I know, I hate days like that, sometimes I have a hundred step day too.