we had this a few years ago, and will try to remember what we did. child was exactly the same never been left b4, parents settled for a few days then left, always saying goodbye. we had had 3 older siblings in the setting! child had a few choice words from the older ones and bought a new meaning to signing songs (but they are other stories)

we made sure the keyworker was in on all of his days, and made time so they could settle the child each morning. the consistency really helped. also asked them to bring him in 15 mins later than everyone else, this did help as was not so busy and distracting for him. he did settle after a week or so and loved the rest of his time with us
sorry not many ideas, you are doing everything esle we did
consistency is something i always say. keep everything you do with him the same for a while, this helps