I agree that he may need more time in a full day environment. Let him cry, do not pick him up right away when he starts crying.Also, talk to the parents about his crying. Make sure they delay their response to his crying a bit too, so he learns that crying doesn't have immediate results. I agree with Blue Rose that you should continue on with your routine as if he wasn't there. Put him in a playpen and let him cry while you are busy doing things with the other kids etc. Put it in the room with the children playing around him, give him some toys and let him cry. He will eventually settle in, some kids take longer than others.
Sierra, are the parents doing things at home that undermine his settling in at your home? Do they rock him to sleep etc. If so, I would talk to them and let them know that it needs to be consistent at home and daycare so that it is easier on HIM (not them). I would give them a deadline to get things in order, or risk termination, so that he can settle and your other children are bothered by constant crying.