The DAS-II is not necessarily lower than other cognitive measures, though it does have a slightly different design than the WISC-IV. There is no obvious reason for the gifted cutoff for the DAS-II to be different from other instruments, but of course, that depends on your district. 129 to 134 would not generally be considered a statistically-significant difference.
In answer to your questions:
1. Most likely this reflects his strengths and weaknesses. ADHD most often affects working memory and processing speed on testing, though, of course, anything can be affected.
2. Depends on your purposes for testing. He's already been tested twice in a short period of time. You won't be able to re-test with either instrument that has already been given for two years from the original test dates (which, I assume, were relatively recent, based on his age). If there is some urgent function for testing, you will have to move on to the SBV, KABC-II, or WJIII/IV COG, or find someone with a WISC-V. Personally, I would lean against it, mainly because he's already had two good measures, which were relatively consistent with each other, from what you've related. Working Memory and Processing Speed don't even figure into the DAS-II global measure (GCA), so they are unlikely to have been factors.
3. This is highly dependent on your area. Some schools use scores as low as 120, while others require scores as high as 145.
