Excess skin? What is this? He's not losing 50 pounds or anything.
If you can't see all of your abs, your bodyfat is too high. I think your bodyfat is probably higher than you think it is. Just because you have shoulder striations and pec striations doesn't mean you're lean enough to have a 6 pack.
Some people have 6 packs, some people have 8 packs, and some have 6 packs that are off center where 1 column of their ab packs (for lack of a better term) is higher than the other column.
Maintaining a 6 pack and the body fat % that goes along with that is probably the best zone to gain mass in...or at least the healthiest.
There are no lower abs. This region is just the first place for fat to store. There's no exercise that can tone up that area and melt the fat away in that particular space. You just need to lose a small amount of weight. The best way to lose weight is to lift weights. Squat variations, Chins, and Rowing Variations are the best lifts for fat loss since they use lots of muscle mass and they can be done more frequently than Deadlifts.