I was always under the impression that no, Oskar would not regreat his decision.
From the pool moment on I had the feeling that he wholeheartedly decided to dedicate his whole existence for Eli. He, in my opinion, will live and die for him. That's all that there is for him after the few years he had lived.
They're perfect for each other. They're each other's support. That's all both of them ever looked for in their existence. Now they have it.
Like other people have said: it's possible and quite likely he will miss his mother.
But going to the point of feeling regreat, I just can't see it.
I actually had never thought of this possibility ... I think. The idea of them being there for each other always seemed like it would be enough to not let feelings of remorse show up. But he definitely misses his mom. As he should.