We were at the same point in the school year when we decided to embark upon a medication trial. I would attempt to get it going immediately, so that the teachers that are familiar with him can give you feedback on any changes. If there are negative side effects, it won't matter since grades are winding down.
If this is not a possibility, I would try the medication in the summer as you suggested. Sometimes it can be a relatively long process to find the appropriate medication (brand, time release) and correct dosage. I agree with you that while he is dealing with transition and testing at the beginning of the school year, this could add an unnecessary level of complexity.
As far as a dramatic behavior change - my son is also inattentive, so the behavioral changes are not so dramatic compared to a hyperactive/impulsive child. I do see a change with his self help, staying on task, even staying on topic of conversation. But, the major change is with school work - which you wouldn't be able to assess during the summer. You might see a "change" but it might be hard to discern if the dosage is optimal.
