J.R.
Hi Charlotte,
Unfortunately there is nothing you can do to change this anxiety. It is what it is,... he's scared because he doesn't understand that you are coming back. Try playing more peek-a-boo with him. It will help him understand that even when he can't see something, it's still there. We used my son's burp cloths and a small toy. To this day, this is his FAVORITE game :) he LAUGHS and giggles those full bely laughs when he plays peek a boo :)
His anxiety was intense - he would cry the entire time I would go to the grocery store and my husband was watching him. his FATHER... my husband is home every day by 430, he puts our son to bed, he feeds him dinner... by no means should he be considered a stranger, but my son still freaked out when I left. For us it lasted a couple months. But i really believe every kid is different on this. My aunt's new baby is turning 2 and still has severe stranger anxiety. they can't even set her down at parties, and she cries if you look at her... my first daughter seemed to sail right thru this phase without any issues - she would go to ANYBODY - which was a little scary too...
Keep reassuring him, increase the peek-a-boo games and he will come around. The worst of it is the heartstrings it pulls on us, seeing them freak out. hopefully your babysitter can remain calm, reassuring and comfort him thru his fits. I think that is very important. I would try using a grandma until he gets past this. She will love him and be more understanding than anybody.
Good luck to you... I hope this phase passes soon for you.
~J.~