Hi Jenn - We were there last fall. I'm sure schedules vary depending on tour directors, but for us, we arrived at Plitvice around 5:30pm. We then dined buffet style with the group at the park. The next morning we loaded our luggage onto the bus at 7:45am and took the very first tram. That tram dropped us off at the trail where the majority of people end their hike. It was amazing. We essentially had the trails to ourselves (and our group) for the majority of the hike by hiking it in reverse. Once on the trails our group spread out and everyone traveled at their own pace. Near the end you start seeing people coming your direction, and soon after it was packed, but that was only for a very short distance at the end for us.
To answer your question, at least with our schedule, you would not have been able to enter the park the day prior. But we felt like we had plenty of time on the trails to stop, take photos, and just take in the scenery. There was a spot after the little boat ride to get snacks, and then a food place at the very end of our hike. Nothing special with the food, so I'd say if he's worried about time in the park, pack some snacks and you can skip wasting time at the food stop and spend more time on the trails. It was really an amazing day - one of my fondest memories of the trip. Enjoy!