I would dig into it. Frankly 28 is an anomaly given the other 3 scores. It might be worth exploring which area of that particular ACT caused the increase to 28 - it’s possible that the material was fresher in his mind as opposed to cheating. Once you have a good grasp on where the differences were then you could certainly call ACT. Most kids move a couple points on the ACT as they progress through school and especially when they are clustered in the lower 20s… but 7 pt. jumps, even in the 20 to high 20s, are rare.