Can a veteran who is unable to use their GI Bill (due to impairment/disability) let their spouse, who is also a veteran, use it?

No, sorry, to my knowledge, you have to have transferred it to him, if you were married to him while you were still on active duty or reserves.

Need to ask that question to the VA. I know children can use GI bill from their parents

As far as I know you can't. It's my understanding that you can only transfer benefits while you're on active duty and you have to have 2-4 years still left on your contract.