I am afraid that I would have to agree with KS and others. Unless the solicitor thinks you have a chance under zambrano that I pm'd you about and I am not qualified to answer that question for you.
Then I really do think that returning to the US is his best option. As Shandy said to apply for DL you will first have to apply for FLR(M) which costs 550 pounds or there abouts. Which is a waste of money because you cannot qualify for it.
However if he returns to the US which will cost depending on where you are flying too about 300 to 600 pounds for a return flight and a spouse visa which will cost you 810 pounds so you are talking bout a difference of potentially 600 pounds but you know he qualifies for it and you won't be wasting your money.
I understand why you want him to have discretionary leave but really if you can find any other way at all I would most seriously consider it. And belive me I so know where you are coming from on this.
Once you get an adverse imigration history and I know from experience because My DIL has one (refused entry) then if he even wants to visit you in the UK he will have to apply for entry clearance first which is going to cost money. Processing times for any visa's will take longer etc etc.
I am not having a go at you but its not an easy road.
Love pads x
Best wishes x