baby has hard time sleeping at night

Caitlin
My 17 week old has been starting to throw fits every time we put him in his crib at bedtime. He didn't do this before, but now the past 2 nights we set him down once he's sleeping and he just cries and cries until someone comes back in to rock him. Do we need to just let him cry? Or is it too soon for the self soothing?