My son is interested in continuing in band in a small college in TX. Not necessarily just marching band, as he enjoys concert season also. The college must offer an environmental science degree also. Which are the best small colleges for band in TX? Thanks!
Look at UNT, University of North Texas, which is north of DFW. I would say UNT is medium sized.
What does he consider to be “small?” What Texans consider to be small might still be considered large to everyone else. 
You can also visit the music forum here on College Confidential, under “college majors.”
UNT is not a small college. 31,000 students is a major university.
How about Trinity University in San Antonio?
Or, if marching is really important, Baylor is half the size of UNT.
Tarleton State - music program and has ES. 11k students, but still feels small. All my band directors went there!
UNT is a long way from being a small school. They have over 35k students.
Look at Stephen F Austin. I don’t know anything about their band but my niece’s husband got a degree in either environmental science or a very similar major and got a very good job in that field when he graduated. My daughter went there for one year before starting nursing school at another college. I was impressed by the built in tutoring and study halls for classes. For various reasons we visited 5 times before she started class as a freshmen. I asked many kids if they could get in the classes they wanted and if their professors cared and made time for them. I got very all enthusiastic positive responses. If it has a major that works for a student I consider it a hidden gem.
Rice is small, has a great music school, and has a marching band. Not to mention its STEM focused for the Environmental Science interest.