The natural progression from retiring from a job in the government tends to be taking up a new job with the government or a government contractor.

This is one place he could look:

https://www.usajobs.gov/

A better path might be to network with former colleagues who work in areas he is interested in. Contract pay in government work tends to be higher.

Outside of government I would say large corporations. Again networking is going to tend to work better here than applying directly if possible.