I had a similar injury, but from a two level floor rather than getting undercut. It happened at the UCD hard courts--where there is a drop off on the edges. Went for a loose ball, one foot landed on the lower floor and the other on the higher floor (the court, effectively). The brunt of my weight fell on the higher knee--luckily it was mostly straight and not bent when I landed. It was swollen and stiff for two weeks. I couldn't run on it, could barely walk. After two weeks, started jogging and was able to play full-go with no lingering injuries about 3 to 4 weeks in. The missed time sucks, but if the injury is similar to what I had, he should be 100% in a few.