Its one thing to not like to study or to be lazy. But to be making silly mistakes on a test would seem to indicate a certain lack of concentration. Look at the test. Does he start out ok and then the mistakes began to happen after awhile - or do the mistakes start immediately. Does he get the same kind of question right one time on the test, and then miss a similar question later on?
Ten year olds typically want to do well on tests. But Margypops had a good question. How does he like his teacher. If he doesn't like the teacher, that could be part of the problem.
The problem is that you have to figure out if he has something like ADD, that would cause him to make these mistakes - or if they are just due to some other factor.
Does he do home work in the evenings, do not allow him any other recreation, TV or Computer until he has finished his studies.Does he get on well with teacher at school and how is his behavior there ?