Where he already has a Dx on the autism spectrum, I think the school would be more open to doing an eligibility eval for an IEP. The verbal cluster of the DASII is relatively limited (and I also find that, depending on whether he was given the preschool or school-age version of the DASII, it is easy to underestimate verbal ability (preschool version)). WM and PS subtests are optional on the DAS.
You have a very reasonable concern with anxiety, and it sounds like, executive functions, which is where I would start from, in terms of requesting a school-based eval. This also meshes with ASD, which, again, will make it more likely that the school will understand and be on-board with evaluation and possible IEP eligibility.
If the school outcome is not to your liking, you can then request an independent educational evaluation, which the district is obligated to pay for, but which you will be obligated to share with them (presumably an outcome you would want anyway). You may be somewhat more limited in your evaluator at that step, but most districts are willing to listen to your choice, as long as they are not exorbitantly expensive.
