"in reasonable charge of the work". The stamp allows you to do this.
As a Civil Engineer, the stamp is important as much of the design process requires code compliance.
For Mechanical and Electrical, where, IMHO, they are creating new applications and technology, licensing is not essential, as in the aerospace and electronics.
Where these other disciplines intermix as in Civil works like water and sewer treatment then some mechanical and electrical and geotechnical work must be performed by, our under the supervision of an Engineer Licensed in those disciplines.
If you have his name you should be able to see if he is licensed through the online lookup.
https://fbpe.org/licensure/licensee-search/