There are so many learning disabilities that it is near impossible to diagnose without testing. If your parents request testing at school, there should be no problem getting the testing. School systems are required by Federal law to respond to each child's need.

There are reasons other than a learning disorder that could be the problem, also. He could need glasses. He could have a marginal problem with his hearing. He could have trouble concentrating because he has some attention difficit disorder. This one is unlikely, but he could be just plain lazy and see no reason why he should work at his school work. My oldest son was an A student through fifth grade and then he decided he did not need to work at it. His problem is that he is a short term thinker. He can't work now for a return later. He is twenty seven years old now and going to college and in his third year. He finally discovered that college is important even though it is years of work with no gain in his way of thinking.

In other words, you can't get around doing the proper testing to find the problem.