Like I said...I’m not a professional...just know what I researched for myself...
If your a green card holder for 8 years or less...then you’d just turn in your green card, Abandon your status, fill in your final taxes, pack your bags, wave, leave the states and have no further tax obligations....
Once over 8 years your considered a long term resident and basically a “citizen” where taxes are concerned and the US taxes you on citizenship and not on residency....
I left the states December 31, 2020 and arrived in Canada January 1 for 40 days before I moved to Scotland . On January 8 I went to the US/ Canada border crossing near me and went into the US border customs and officially turned in my green card....which means I filled in the Official paperwork saying I was of sound mind and wasn’t being coerced into doing it...and Officially Abandoned my green card. They sent the paperwork in and in April I received a letter dated in March acknowledging they had received it and i was no longer a green card holder....they also tell you to keep the letter and travel with it if you return to the states so that if you get hassled trying to enter the states that you can show that you willingly gave up your greencard....
Turning in greencard just lets immigration know you’ve ended your status....has nothing to do with Uncle Sam.....that’s a different can of worms.
So, because I had no US income for 2020... I need to fill out a 1040 NR form for my taxes this year....and form 8854 to end my tax obligations to the US.....what is form 8854 you ask..... well, it’s basically a form you fill out when you expatriate from the states showing your world wide net worth on the day you leave the states and you also have to show that you’ve been tax compliant for the last 5 years and filed your tax returns and payed your taxes. Uncle Sam figures if you’ve lived in the states for over 8 years and amassed any wealth it’s because you lived there and he wants his piece of it and will charge an exit tax if he can. Now, you have to have over 2 million in assets before he charges an exit tax....which I certainly don’t have....lol.....but form 8854 asks you to list everything you own, any pensions you will eventually get....and stocks, iras ....so you basically have to show your net worth on the day you left the states and they will determine if you need to pay an exit tax....
Once the final tax year is filed and form 8854 submitted....that should officially end any tax obligations to the US and you have expatriated....
But these forms have to be filed in a timely manor or else your talking fines.....
Once all this is done...I should still be able so collect SS because I paid into it enough and even though I am no longer a green card holder I should still be eligible to collect it ....
Like I said... I just researched for my situation...yours is completely different....and if you’ve been filling out taxes for him each year and declaring his UK income to the US.... that’s a different can of worms