I think I'd go for helping him make his sadness about the end of the school year explicit, e.g. helping him make a list of all the things he'll miss about school over the summer. This might make it "worse" in the short term, but there's huge benefit in being able to recognise one's own feelings for what they are and own them rather than suppress them (wish I could have learned that at his age!). It may also help in finding ways to have fun in the summer, but I wouldn't push that in the same conversation.
