6, 7, 8, it's no big deal. Kids phase in and out on what they will do. Some days they are "on" and working hard. Some days they phase out all the time.
Some time ago one of my kids played on a pretty low level travel team and one of the boys was still a daydreamer out there at u10. His dad actually played for Liverpool for 2 years, and he had an older brother who played for a major D1 school. So a big soccer family. But, their youngest was a nice kid who literally could be picking daisies out there as a u10. Yet, dad was a great example for us. When he could get to a game and his kid was phasing out -- he would say "xxx what can you do to help?" Totally great.
The reality is that it is a maturity thing more than anything else, and there is very little that you can do to address it other than to make sure that he is having fun. Eventually he will catch on that it is fun to work and try hard. Although, I would guess, that you still have the phase of working hard to no particular end to go through.