Welcome to BW!! And congratulations on starting BW so early!! You won't regret it!!
As JAY3 suggests, gas could be the problem. My lo went through the same thing, and we realized he was a silent refluxer. Have you thought about this possibility?
Have you tried shpat?
If he has silent reflux, the patting on his back makes things worse. But, for us, patting on his bun really helped him settle down.
xxx,
Alexa