I think it would be a good idea to tell him that if he can't figure out a question within a few minutes, move onto the next one. My daughter takes forever with these tests too and I have told the school to cancel the test if it's stressing her out or if she is taking too long.
It sounds like he's suffering from Analysis Paralysis, and going over questions too many times trying to figure out how to get one of the answers in the multiple choices. Tell him if he doesn't understand the question or hasn't learned the concept yet, guess and move on. The goal should be to figure out what he knows, and if he's getting some right by eliminating obviously wrong choices and then just getting lucky, the test isn't really measuring what he knows.
