Hi, I have read what you have answered and really do believe positive encouragement is the way forward. Make study sessions of a reasonable time with positive activities to look forward to at the end of the time. Praise improvements and keep a close check on grades and homework being completed and handed in. Some children simply do lose enthusiasm at school as they may be bored and need to move on to the next class.
Don't push the enthusiasm to complete work just the fact that he does it will warrant praise and then reward when good grades come through. This could well be a teenage phase that will pass, try not to make too much of it just continue to encourage and provide support with homework.
I hope this helps! Good Luck!