I would go with excitement about the opportunity - this decision to summer school the gaps could change the rest of his school maths experiences!
I'd be like "ooh I have some exciting and interesting news for you"
Show results of test
Option A - do summer maths one on one, and then get to do algebra 1 next year and therefore be bumped up for the rest of school life
Option B - go combo class next year as planned and therefore understand that you need to comply with all requirements for this class, even though you know 3/4 of the work already now.
then end with "I know what I would prefer you to do; now you need to decide for yourself."
make sure he understands each option, the time involved, the commitment involved and the expected outcomes.
then with grace allow him to chose and therefore direct this himself.