High oxalates are not common for Dent.
However, sometimes even levels this high can come from dietary sources, but you could have him tested for primary hyperoxaluria.
It is very important to minimize oxalate in diet. Did you review it with the nephrologist or a nutritionist?
High doses of vitamin C can also give you high oxalate, make sure he is not getting that.
We also have a center for primary hyperoxaluria if you would need genetic testing.
Where did you get with this so far?