If he has been doing WebForms instead of MVC, he's probably not up on all of the basic table stakes client side stuff like BootStrap and JQuery.
I know at 41 years old, if I want to stay in development and not go into management and command the salary I want, I can't be complacent. The minute that my company stagnates, I must find another job. That means for me, being a full stack .Net developer:
1. Web - Angular, JQuery, CSS, TypeScript, and Bootstrap 2. Server side web - Asp.Net MVC, WebApi, WCF 3. Knowing how to speak the language of an architect (DDD, Design Patterns,everything that Martin Fowler writes) 4. Database theory and maintenance and EF. 5. Testing - front end and back end automation testing.
I'm not bragging, I know lots of developers who can tick off these checkboxes. If you're not willing to aggressive learn, this isn't the field for you.