Sporting's coach, Peter Vermes, replied to the rumors by saying Brentford's offer wasn't even worth discussing because it was way too low.
But even if Dwyer left the US now, he could still gain citizenship by 2017 and be eligible. He has a green card, and the requirements to gain citizenship are to spend at least 2.5 years of the 5 years wait period in the US, with no period abroad being longer than 6 months. So as long as he flew back for a visits, he would be able to fill the requirement, since he already has over 2 years in the US since he got his green card.
Not saying he will ever suit up for the USMNT, but just noting even if he goes back to England it's still possible.