I am happy for you that the parents are supportive and you are willing to work with him.
Unfortunately in my case of a very structured daycare I had a child who did not like structure, show any respect for adults, attempted to hit me the day before I ended care and parents who did not seem at all surprised by my reports of his behaviour or want to work with me to better the situation.
There were red flags from the get go with him from the initial playdate with his father but I chose to look past them thinking it was first day nerves. I had 2 kids of my own and four daycare kids as well as this boy and after MUCH discussion with my assistant and the teacher who came in regularly to provide preschool education we all agreed that he was just not a good fit for the group as a whole and that his behaviour was negatively impacting the other children. (One dkb completely clammed up and kept away from the group when my angry boy was with us and he absolutely shone when angry boy was not around.)
We did work hard with him over that month and I really agonised over the decision because I hated to let the parents down (he only needed 4 months care until JK).
For me in my situation I made the best choice for my daycare. I felt bad about it for a few weeks but I knew I couldn't have done more. All the kids in my care flourished after he went and I learnt a valuable lesson about taking on part-timers.
I hope it works out well for you