Chiming in with my 2 cents on this discussion:
If Jack came back, I think Ellery would feel like he is keeping Ennis from being with his first love, and he would opt to end the relationship with Ennis so that he could go be with Jack. That said, I think Ennis' emotions would be in turmoil for a while and he would need some time to think and clear his head. Like others have mentioned, Ennis has had a lot of positive changes over the past coupla months with Ellery and Jack must've changed too over the period he was gone. That to me, does not mean that they would've lost the 'chemistry' they had before. People grow and change in relationships and the person you fall in love with will not remain exactly the same forever, there will be some changes.
That said, if Ennis spends time with Jack and realises he still misses him so much and would want to be with him, then he is indeed in a pickle because he loves Ellery as well .. if after spending some time with Jack, Ennis realises he loves Jack, but his place is with Ellery, that would also be a difficult situation to handle.
This is a toughie .. it raises another question in my mind:
Can you be in love with more than one person at the same time?