S.W.
This is not a test to determine learning disabilities. It is used to measure language ability in people of all ages who are English Language Learners, people whose first language is not English. Schools need to measure children to decide how to best teach them. If your son needs extra help in learning English, he can only get it after he is tested to find out his level of skill in English right now. If your school is like most, your son will be given several tests at different grades, to help the school know if he is learning and if the teachers are doing their jobs.
http://www.assess.nelson.com/test-ind/wmls-r.html
"The Woodcock-Muñoz Language Survey Revised (WMLS-R) includes a quick and easy-to-administer screener (four tests), and a more-comprehensive 7-test battery, both designed to measure language proficiency of English language learners (e.g., ESL students). The seven tests that comprise the WMLS-R measure the critical areas of listening, speaking, reading, and writing."
These same test can be used to decide which children to put into Gifted and Talented programs because of their advanced abilities. My daughter was given the Woodcock Johnsont test and scored very high and so was put into the G&T program. It is in no way only used to test kids who are suspected of having learning disabilities.
If you are concerned, though, the best person to talk with is your son's teacher. He or she can tell you what their reasons are for giving this test to your son.