Maybe the in call volume on your sons phone is set to low?

 

Exactly what the previous reply said...try turning the volume up while on the call....it won't work if you go to settings then sound then volumes...you can't make changes to in call volumes thru there...only while on an actual call

 

My room mate and I also got LG Motion 4g,....having the same issue,.... his in call volume is down to almost nothing,.... did all the volume control in settings And in call volume adjustments,.... no success,..... even did a factory reset,.... still the problem persists,.... I concluded that it must be a hardware problem,..... will go to store today (not expecting any help)......... anyone any thoughts?

 

Went to a "corporate" MetroPCS Store,.... guy checked it out and offered a warranty exchange (original purchase was Jan 2013),....warranty period=1year,.....since they didn't have any in stock, sales person assured a exchange 3-5 days,....... problem solved.

 

I had this same problem, happened with In-Call volume (had to adjust for each call) and also when headphones were plugged in (volume, instead of automatically being lowered to protect my ears, it would jump to full blast volume). I found that the problem was actually firmware related.

I downgraded to Firmware Version C and it fixed it. I'm not sure what firmware I was on when the problem started, I think it was F. Maybe try the unbrick guide to return to C? There are tutorials and videos that are relatively easy to follow