A person is allowed, without a permit, to impound a certain amount of water. See below. http://fisheries.tamu.edu/files/2013/09/A-Pond-to-Call-My-Own-Understanding-Water-Law-in-Texas.pdf
However, if his impoundment floods your property, then this is more than likely a civil matter.
I would contact the federal Natural Resource Conservation Service or Texas Commission on Environmental Quality office for your area.
As always, it's better to try and work it out with neighbors before involving lawyers."The Texas legislature has
encouraged the development of certain
types of small ponds and lakes by
exempting some landowners from the
surface water permit process. It is often
called the “stock tank” exemption. Section
11.142 of the Texas Water Code allows a
person, without obtaining a permit from
the TCEQ, to construct
“…on their own property a dam,
pond or reservoir storing not more than
200 acre-feet of water for domestic and
livestock and fish and wildlife purposes.”