I don't know if this is any help ... -- for preventing boredom with practice.... these things worked for mine:
a list of clear goals when child leaves the lesson -
for example: play bar 5 3x/day -- (and WHY that bar(s) needs repetition (what is the child meant to accomplish - correct rhythm, notes, dynamics, phrasing etc)
practice charts (with clear goals, number of repetitions required) are good
not too long on one piece - max 3 - 4 weeks. Move on even if the piece is not perfect - otherwise ..boredom sets in
charts are good for learning scales/arpeggios for exams
child 'controls' of their practice time - ask him - 'how long do you want to practice - 3 min - 7 min?' give them a timer and INSIST that he stops when the time is up. he will be probably be surprised by how quickly the time goes by and want to do more.
Maybe structure the practice time.... (again - use a timer - for example -- .. 5 min on piece A, 7 min on piece B, 4 min on piece C .. & thats 14 min practice already ... nevermind the scales! (another 6 min)
children (and adults... like me!) have short attention spans .. one rule (that I heard) for kids is practice time = age of child (in minutes) + 2. For your son - this means 13 minutes - no more.
lastly - sit with him and listen when he practices .. this lets him know that you think practice is important! even just 2 minutes is meaningful!
maybe these things might help your son be more motivated? my own dc did grade 5 piano on just 20 min/day .. age 10 ..I say this only to emphasise that it's possible to do very little practice (but highly concentrated) and do well. As for grades... they might mean a lot when your child is younger but once they leave school - nobody really seems to ask... although actually - Grades 6,7,8 yes - they are theoretically useful in terms of UCAS points .... but this is an expensive and time consuming way to get UCAS points unless you are really into doing exams