OP here, thanks again to everyone for the very helpful posts. 8:49, I also suspect our son has very slow processing - he is quite slow in organizing and verbalizing his thoughts. Thanks for helping to bring attention to some of these issues for us. I think the only thing that makes sense now is to get a good comprehensive eval to see exactly what we're dealing with. Also, it's good to know that your DS is getting the services and accommodations he needs under the speech label. I think we'll ask to stay put with the DD label then revisit the label once we see the results of the testing.
PS - Thanks also for the resources re MERLD