Lower back? Or upper back between the shoulders? I'm assuming a Puch road bike, but they made a BMX and other stuff too.

On a road bike, sitting too upright is a mistake in my opinion. That will put more weight on both your shoulders and upper back as well as the saddle. For me, leaning forward lets the power my legs are developing lift some weight off the saddle and the fact my body won't bend much more takes some weight off my wrist and arms.

As for his pain, I don't know. But when doing longer rides than your body is used too, muscles will squawk for awhile till they get used to the new norm.