What career options are open for a software engineer apart from an MS or MBA in its fields? Can he pursue a career in environmental science fields? If yes, then what would be the path?

Any specialization, whether Civil, Mechanical, Environmental, or Automobile, relies on technology/software to make informed and meaningful judgments.
Mechatronics and the Internet of Things, for example, are two developing fields that are making autonomous automobiles a reality.

Groundwater modelling and even soil mechanics analysis are examples of software applications in civil engineering.
Environmental sciences mainly rely on software to crunch natural resource data (Air, Water, Soil, Food) and analyze important indicators in order to create meaningful future forecasts or even manage environmental dangers.
GIS is another important area where software may help (Geographical Information Systems or GeoInformatics). That is, using meteorological data to identify disasters, floods, earthquakes, and storms using maps.

Remember that Big data is the common thread that runs across everything. Statistical software provides the capacity to examine Big Data. If you’re excellent at programming and data analysis, you might work in any of the multi-disciplinary disciplines listed above to address real-world problems.