My husband & I separated Sept 15. The letter clearly states he can only continue to claim if my Son is in his custody but he is not anymore. I'm just wondering if I have to go through the entire process of getting him declared disabled all over again to have the letter issued in my name.
To claim the Disability tax credit you need a Form T2201 for your son. If he already has one, it will be under his name and not anyone else's. So you should be able to transfer the credit without making any changes.
Here is some more information on how to claim the disability tax credit transfer.