Loy, I have no experience with the WJ achievement, however DD did take the WIAT for the results we needed for DYS.
The psych. gave us the option to wait until she was in 1st grade since there so many additional subsets that start at that grade level. We wanted the test done before 1st grade to qualify for DYS and for advocating purposes, so we tested her when she was in Kinder, but the tester gave her the subsets that start at the 1st grade level. In the report, she had two data sets, one for kinder and for 1st grade. Her kinder reading (which really only includes early reading) did max out and no broad reading was not calculated due to the missing subsets. However, she able to calculate a broad reading score for the 1st grade results because she was given the necessary subsets. Same for the broad math; you need to ask the tester give the additional subsets to calculate the broad score.
As far as figuring out where he is academically, I'm not sure if either the WIAT or WJ is a true indicator of how he will be performing in school, unlike the explore, etc. I get a better idea of that from working with her at home and what I have observed from school with my older DS8.
So if you wait a few months to test him, then you shouldn't run into any problems as far as the ceilings/scores he need for DYS. If you test now, ask the tester to go up a couple grades. HTH!
