I think this is normal for kids in the first term!!
Marks in the 50s aren't that bad. The percentage thing is a bit strange. It is notionally out of 100, but many universities really mark out of 80 for all practical purposes. Some lecturers may give higher than that, others simply won't. So one person's 80 is another person's 100. Confusing and odd, I know. To make it still more strange, some departments only give set marks on a scale, so while it looks like it's out of 100, in actual numbers the essay will be graded a 52 or a 55 or a 58.
Marking is more impressionistic, especially for essays, than you might think - but hopefully experience and moderation and external checks mean that things are consistent.
It sort of evens out because of the grade boundaries per course and the way that overall grades are calculated. To give you a rough idea, in most places 70-80 is a first, 60-70 is a 2:1 and 50-60 is a 2:2. However, the final degree certification will also depend on the distribution of the marks, so actually, a certain number of courses above about 68 will be enough for a first.
On a more personal note, I doubt that yelling at your son will do much good. He's away from home for the first time and enjoying a wonderful experience. I worry a lot that kids now are under so much pressure that they don't actually get the chance to experiment (personally or intellectually) at university. And if they lose that, they lose something very valuable about the whole experience, because they can become so obsessed with marks (my colleagues and I jokingly call them Marksists) that they actually turn into the kind of boring, mediocre students who keep their noses clean and end up with a high 2:1, but without any flair, personality, creativity, or originality. They are transformed into identikit units for the graduate workplace, like units on a production line - and there is so much, so very much more to life than that. I also see far more anxiety in these kids now than I did 10 years ago, and this makes me worry a bit for their long term mental health.