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.
It's a crap-shoot on which technology is going to take off. Microsoft was pushing heavily that MVC was just another way to do things during the events I went to. I tried to push the boundaries of what my company let me work in, and that just happened to not be where the market went.
In hindsight, keeping my eye open on the job market is something I should have done more.
Seriously it's way easier than webforms so if you've got that much webforms experience, you'll have absolutely no problem picking it up. Within a week you'll know 95% of what you'd need to know on most tech interviews and you'll do fine at your new job. Stuff varies so much company-to-company that MVC will not even be one of the top five differences.
In my previous response I was thinking more you were looking for some big change into something very cutting-edge. That would be a more difficult proposition. But if you're just looking to go from webforms to MVC, or even to a Java-based infrastructure, it's not a big hurdle. Like others have said, the HR filter will be the biggest problem. There will be an age bias too, you'll just have to deal with that.
FWIW I don't think you should expect a paycut or a cut in "rank" either just because of changing technologies. You'll get caught up on the technical side very quickly. Outside of extremely technical companies, "rank" is far more about knowing how to get through a release cycle coordinate teams than it is about specific technical knowledge.
All that said, just be warned, working on websites in MVC (or really any technology) is really no more interesting than Webforms. (In fact webforms may be more interesting because you get to invent your own way around its inadequacies).
I ran the software dept for a small consulting company and yes you'd probably never get a call back from me if you were applying purely as a developer. While we have hired late 30-somethings developers, it's usually because they offer some fairly unique tech skillset (i.e., a few years of production-quality FPGA experience, not a couple months dorking around with "whatever hot new technology" on github--that's something I look for in a fresh-grad). So I think for a purely tech role, beyond getting another job at a similar company doing similar tech, you're a long shot at best.
But beyond that, the one thing that would case me to look at your resume is if you were wanting a leadership position and had the corresponding soft skills and experience. It sounds like you do, so highlight that, and work on it. The advantage there is that management skills: talking to stakeholders, setting expectations and timeframes, finalizing deliverables...are all applicable no matter what the underlying technology. So there's no reason you'd have to limit yourself to ASP management roles.
My previous company had lots of young developers doing engineering with lots of new technologies, but they'd have never delivered a thing had they not been guided by some experienced devs who actually spent most of their time in Excel (or trello or Jira or whatever we happened to be using to manage the particular project).