I'll go against the grain here. It does depend on what your son is using it for.
Pluses for the 928 though: much better camera (OIS, bigger sensor, higher resolution, xenon flash, hardware shutter button). Video will be much better with OIS and it has three mics as well. Glance screen available. The speaker is LOUD. IIRC, it has a special amp for it. None of the other Lumia's I've tried come close.
Pro for 735: microSD slot, could be good for tons of music onboard.
Look for a used 928 around $100-120 range and it's a great phone.