Sounds like he's going through the Wonder Week where he realises you can go away from him and he cannot reach you. Do a search for Wonder Weeks on here, and also on Google and read up about it, it won't stop him crying, but it will reassure you that it's normal and will only last a few days.

If you google it, follow the link to the cyh, and from there they have a link to the actual book site which has lots of great excerpts and the all-importnant "calandar" which show you when the WWs occur.