Jessica, I would say he might just be experiencing separation anxiety, which peaks anywhere between about 10ish mths- about 2ish yrs. My DS has recently started screaming when I leave ofr work too, and I leave him with DH/SIL/mum. So it's not that he's with someone unfamiliar (especially when with Daddy, he ADORES his Da!), it's just that I'm leaving him. Once I'm gone I'm told he's happy as Larry, but it's just the actualy act of seeing me leave the room that sets him off. Sometimes I can't even go to the loo without him crying!
I would say he's happy but just experiencing a normal developmental stage. If you could, maybe putting him in another day might help with the anxiety?