Not sure if this is still helpful as your son might have already decided (or taken) one test. But my DS13 took ACT this year (SAT last year) for the purpose of applying to summer programs. He did ACT with writing. We didn't let him over-prepare. But we bought one of those prep books which had a few writing topics from previous tests, and we let him practice coming up with an outline quickly. Namely, when he sees a topic, we give him a few minutes to decide what points he wants to write about and in what rough order--but not really doing the writing. This way the practice doesn't take much time but he is not going in completely cold. When the test score came back there was also a brief evaluation of the writing section.
