As others have mentioned I found the best way was one week of transition. Monday 1hr (not parents) Tuesday 2hrs, Wednesday half day with lunch, Thursday stays through nap (parents on call for pick up when child wakes), Friday full regular day.

Also, have the child arrive after all the kids have come for the day. This way they don't have random adults showing up and disappearing and they have other, calm children there to see.

Do you think mom would be open to changing the transition plan? Tell her that he is comfortable and exploring by the end of his time with you but he is regressing and starting over when he comes back days later (it sounds like he is not coming daily right)?