This is a fan-made costume.
Designing and wearing custom Mandalorian costumes has been quite popular for years, especially after the Clone Wars series brought in new, canon Mandalorian armor beyond the original Boba/Jango armors from the movies.
While I have no direct evidence, that person is likely a member of the Mandalorian Mercs, a costuming organization that frequently partners with the 501st Legion and Rebel Legion to invade sci-fi/comic conventions and perform charity work.
Most people building Mandalorian armor of that level of detail start off their hobbying at The Dented Helmet for resources, guides, and connecting with expert prop-makers. They usually connect with the costuming groups from there or vice versa.