I am not, though your question prompted me to poke around a little. I will say, though, that
1. Every test is different, so a certain amount of minor variation in standard scores is to be expected. Grade and age equivalents may vary even more (keeping in mind that they are not really good measures of progress at the best of times).
2. There are multiple levels of ISIP, just as there are for many other tests. Do you know what level he was administered? I believe grade 4 begins a new test version.
3. DIBELS is relatively limited in its scope, being focused more on decoding than comprehension. I can easily see scoring much better on DIBELS than STAR.
Technical Report from the publisher, on ISIP advanced reading (grades 4+):
http://www.istation.com/Content/downloads/studies/ar_technical_report.pdf
