Hi Sabine,
Sounds like he might be overtired. That has happened to my LO. The moment I would put her down in her crib she would cry. It was out of character for her. She doesn't usually do that. So, I sat with her in my rocking chair to calm her down for about 10min, then I would put her back in her crib, rub her forehead and tell her it was doh doh time (french for bed time). Then she would smile and eventually fall asleep. I would hear her mantra cry on the monitor.
I hope that helps
Dree