He is a great Romanian violinist based in Paris. He’s a real interesting player in the “gypsy jazz” world.
That style is really a modern interpretation of Django and Stephane’s style, kept alive mainly in the Sinti/Manouche communities in Europe. I would say that the style of music is treated quite like folk music (certainly by guitarists) in terms of the improvisational language, the way of learning and the repertoire but at the end of the day it’s jazz!
Violinists like Costel have a distinct swing feel and style of playing, just as close to other Sinti/Manouche and Romani fiddles styles as it is to Grappelli’s playing in my opinion.