I wouldn't worry about the short term memory score, either. I think those scores are often affected by how interesting the children find the task. They don't remember what they don't care about. DS got a working memory score of 47th percentile on the WISC4 (boring task) and was diagnosed with ADD because it was so much lower than his other scores. But he got 99.9 percentile on working memory on the SB5, because he thought the task was really fun. So I took away the ADD diagnosis without consulting anyone. I'm guessing that the short term memory on the SB 4 is a similar kind of thing.
And I wouldn't recommend doing another test. These scores are giving him what he needs right now and helping you get a sense of where he fits. If at some time they don't, then you can revisit the issue.
