Unfortunately, for many people, the skin never tightens up. It is sadly the case that not ALL of the effects of obesity are reversible with weight loss -- and stretched out skin often falls into this category, especially the older we are when we lose weight (and the more years we were overweight). Surgery may be an option. There are no exercises that can solve this problem, and unfortunately the area that you're talking about can't even be somewhat improved by building muscle as there's no "bulkability" to that particular area.
It's probably best to focus on the health benefits of losing weight and just accept that our bodies sometimes do bear the scars of what we've done to them over the years.