My advice is to keep it as loose and un-constructed as possible and stay away from workbooks and actual lessons over the summer. If he doesn't realize he's learning or supposed to be he may get more out of it.
1) Have him write in a journal daily, about what he is going to do or did that day. Have him write at the same time (or times) each day, and consider having him keep it up in the fall. When my daughter did this I was amazed at how much she would write, sometimes page after page after page on a day I hadn't thought there had been much going on ; )
2) And, have him read books and write book reports on them, reading helps in learning proper grammar and sentence structuring, so it will actually be instructing and helping him in those areas. If there's a summer reading incentive program in your area, (many public libraries offer them, ours offers one for students through 8th grade,) it will make it a fun goal to encourage him to read more, too.