You've received some great ideas already!
A few more thoughts...
Possibly ask
him to look at his letters and tell you which ones he is most pleased with, which ones he'd like to redo... then support him and accept the level he sets? This may help him feel in control of his education and level of mastery and provide the benefit of ongoing sense of ownership of his decisions.
Possibly set a timer to ten minutes and then stop, no matter what has been accomplished? This may help him develop trust that when you say 10 minutes, you mean 10 minutes. (On the other hand, if your rule is work until a lesson is completed, then you may wish to remove the reference to "10 minutes" as an estimate... he may decide to move along more quickly to completion.)
Possibly ask what he would like to learn/see/do/experience this summer and help him achieve that? This may help him learn to see you as supportive and also help him understand that a relative weakness in one or more areas does not establish a ceiling or limit as to what he can learn and achieve in other areas.
He may need the above skills and internalized beliefs to stay motivated throughout school (and life) and may also use them as a foundation for his own self-advocacy efforts in higher grades.
Possibly you may find the book
A Parent's Guide to Gifted Children to be helpful?
