I don't know a thing about the
Thornless Honeylocust (Gleditsia triacanthos 'Shademaster') since it doesn't grow this far south but if the leaves are yellowing and falling I'm wondering if the issue could be insufficient water reaching the roots? Has it been very hot in your part of the country, have you had much rainfall since planting the tree?
Hopefully someone with knowledge of that beautiful tree will pop in with advice.
