It sounds familiar to me. My girl was like that when we first started her in full day childcare.It's something that they will slowly wean off. What i did was i got her teacher's help to shower her with praises when she stopped crying or wasn't crying too hard and then i will pass her teacher some little treats like biscuit or stickers for her teacher to 'reward' her with.
I guess she felt 'special' as in she was the only one getting treats and so she slowly started getting better. When she started making friends, enjoying their company and enjoying school, she stopped complaining and crying.
Hope this works for your boy, Hang in there! It will be over before you know it.