I don’t think it would not be worth it. The Studios will be open until 5pm, and Islands is always open at least until 8 or 9. Most of the crowds in the Studios do not start until later in the afternoon, when many come to be inside the park to wait for
HHN in holding areas that are set up as it gets closer to 5. And if you are staying onsite, take advantage of the early entry to the Studios Potter section and I think Minions too. Then finish up the other fun stuff you want in the Studios, and either ride the train by 4 that afternoon, or walk to Islands and have fun over there until park close. The other morning, start in Islands to repeat and catch anything you missed the night before. Take the train, as a different scene is seen riding this way, than riding it into Islands on the night before. Have lunch, and spend all you want until 5, then walk out, and enjoy dinner in CityWalk before leaving or just leave. There is lots to do in CityWalk as well. Movie Theater, Mini Golf, and lots of shops and food places.
I hope HHN does not scare you off enjoying a weekend at Universal. Also know that anything scary is not out in the Studios and props are covered over, so if your son is not fond of scary, there will be nothing in the park to scare him.