If he's from TSC, then he is a hatchery-quality Dutch bantam, not an OEGB mix. Many (likely most) hatchery Dutch lines
have been outcrossed to OEGBs to improve fertility and vigor in the past, but that's no different than a breeder outcrossing to improve their line in some way. Chickens are not pedigreed like other selectively bred animals are; instead breeds are distinguished by their appearances. Outcrossing maybe produces mixes in the first few generations, but once the birds have been bred back to the original breed or have been properly selected for breeding and return to having the original breed's traits, they are then just considered that breed again. If that was not the case, then we would never be able to introduce new varieties to any of the established breeds.
In short, it would be far more accurate to refer to him as a hatchery-quality Dutch bantam than as an OEGB mix if he's a TSC bird.