My 9-month-old son has the same thing. We also had an u/s done when he was a couple of weeks old because his pediatrician couldn't see the bottom of the dimple. The results from his u/s came back normal, so we quit worrying about it. It doesn't seem to have any effect on his development or comfort. Like yours, my son has just recently figured out how to get up on all fours and rock and he will stand and bear weight with some assistance. At this point, neither his pediatrician nor I have any concerns about his development. He seems to be well within the range of normal for all of his milestones.
We just had our 9-month appointment yesterday and while the doctor noticed that he does still have a sacral dimple, she didn't feel that there was any need for further evaluation since he doesn't appear to have any issues with it and his earlier tests came back fine.
At this point, his dimple seems just a cosmetic thing. It's just another little crevice to clean every time he gets a diaper change. The dimple is definitely less pronounced than it was when he was first born, but it is still there.
Hope that helps. You're obviously an experienced mom with 3 kids of your own, but I've noticed that in most cases, my gut tends to be right when it comes to the health of my kids.