I also have this problem or should I say I did.......But now it is much better. It took a good 3 weeks for my 12 month old girl to adjust and she is here full time. Because he is there part time it will probably take longer....the best thing you can do is encourage him to play and ignore him as much as possible, engage with the other kids so there not feeling stress or annoyed he will come around just give it more time and be consistant eventually he will get that he's ok and come around, my 12 month old is happy go lucky so Im glad I stuck it out.