Can you see the bottom of the dimple yourself or is it too deep - I am guessing that is why they gave an ultrasound?
Are you afraid that he may have a tethered cord or something because they failed to properly assess the thecal sac? I know tethered cord can lead to problems with the bladder and bowel, but as far as I know, these problems relate to loss of control of the bladder/bowel rather than constipation.
How are his gross motor skills? Is he walking/running/jumping, or close to doing so? How is his balance? His general strength?
If these are all ok, it's unlikely that he has a problem with his back or that something to do with his back is affecting his bowel - you'd be seeing other problems too rather than just bowel issues, if that makes sense.
I am not a doctor, though. Best thing to do, if you're worried, is get it all properly checked out. Ask for another ultrasound now he is bigger.
My oldest has a 'forked gluteal cleft' (the top of her bottom is shaped like a Y) which can be a sign of a tethered cord, but she doesn't have it and I was told when she was a baby (and I was worried) that thousands of babies have bottoms like this and nearly 100% them are absolutely fine. As my daughter has grown it's become more than clear there's no problem.
My youngest has a sacral dimple but it is definitely closed and you can see the bottom of it. I was told this when she was born.
Neither has ever suffered with constipation. Plenty - LOADS - of perfectly healthy children do, though. Do they know what is causing his growth to fall behind?
P.S. Both my girls also had lip and posterior tongue tie, which are defects along the midline that are closely associated with funky looking bottoms and sacral dimples. Not entirely sure how it works but it's apparently something to do with how a layer of cells in an embryo folds/doesn't join up quite right - nothing wrong with either of them, they're just a little defective, but not enough to have any issues!
How's your LO's tongue?