I would say that perhaps his problem is... positional play? makes any sense?
How overrated I chess tactics trainer

Are the games played here his? The reason I ask is that the blitz rating is down nearly three hundred points from its peak.
To me, that shows a kid who needs a break.
The tactics rating is not the same as a rating earned in competitive play. It is good that he has a high tactics rating. As you said, there are many other aspects of play.
Is his rating correct? Probably.
How often does he play rated games? Does he have friends who play? Does he enjoy playing?