Belly sleeper?

I am pretty sure my 8 week old is a belly sleeper. He does for naps but not at night. At night he is constantly waking up and I think it's from his arm movements etc... we have a monitor that has a sensor pad to detect when there is no movement and it sends an alarm off. I am still terrified of letting him sleep on his belly. His older brother was a belly sleeper ad well but I think he was a little older when we let him sleep on his stomach. Any advice? Thoughts? Am I over reacting?