My 16yo has learning disabilities too - although different ones and no anxiety - well social anxiety but not generalized anxiety. Not yet - very strong family history. Anyway - he is totally fine home alone during the day. I think you can cross that one off your list
. Am I reading right that your plan is to continue the home schooling online? I do not have experience with that. My son would never manage that in a million years but all kids are different. If the plan was public school in August I do have some experience to share with that. Your son would qualify for a 504 plan at least and possibly an IEP depending on test scores. The 504 would provide needed accommodations and supports. IEP's provide content modification if needed - and therapies. If you'd like more info please let me know. If you pm me the state you live in I can give links for agency contacts in your state that might be a good resources. These are legal requirements in public schools. Not sure if your online school is public or not?
Your son has a lot of challenges but a lot of assets with his talents and athletic gifts. That is a big one at his age. He sounds like a great kid with a pretty full life actually and you are a great mom.
I also suspect that his time with his dad will be fine. Once the transition with the divorce and separation and parenting plan is settled they will find a routine - and it probably won't be one you like and/or X won't follow through on agreements but that is life unfortunately.
And don't feel bad about what he can and can't do. My kid could not spell his way out of a wet paper bag. I mean - he really probably can't spell paper bag and if he does he couldn't read his writing. He is accommodated by not being graded on spelling. He needs a job that does not require spelling - or make sure he has a secretary. Lol. But guess what?! He won an award for best GPA in his intro to business class! Had the right teacher for that one!!