It's a great question 'should summer be for fun?'
I would submit that in the best of all possible worlds - there 'should' be a reasonable amount of challenge during the school year, and there 'should' be easy access to playmates who are peer both in age and intellect. If both of those conditions are met, then sure, summer should be for whatever the individual finds fun.
Sad to say - for most of our kids those conditions are not going to be met. Then summer is a great time to build some of those important character building skills.
Next question - who's idea was it to have your son take a writing class? My son attended SIG, a gifted day camp around that age, and they were very strong is saying that the kids should pick classes that interest them. Of course I didn't follow their recommendation - I made a deal with my son that he could pick 2 classes, and I would pick one (and yes, it was Writing!) Lucky for us, both years the Writing classes had terrific teachers and were quite easy and fun.
So if you are the one who picked, are you feeling guilty? If so - cut that out! My guess is that you had very good reason for choosing writing - perhaps to even up some abilities to prepare for another gradeskip if it is ever needed? There's nothing wrong with wanting a little 'insurance.'
If possible, observe some of the classes in Math and Science so you can see if they would be 'too easy' if he decides to take them next year.
If a child is HG or PG, they may have to choose a weak area to get anything out of a class aimed at MG kids.
Anyway, I would talk to the teacher and say what you told us. Sometimes the teachers will suggest modifications to make the class a better fit (shorter assignments, or dictating the assignments to you for you to type, or allowing the child to keyboard)
Love and More Love,
Grinity