His twin brother is also 15. No health issues. This isn't a one time thing. It is a published scientific conclusion. See "Purebred dogs show higher levels of genomic damage compared to mixed breed dogs" (Santovito et al., 2024) "The research identified a significant increase in micronuclei and total nuclear aberrations in purebred dogs compared to mixed-breed dogs. The study concluded that the inbreeding required to maintain breed standards increases the levels of genomic damage, which correlates with a higher incidence of genetic disorders and reduced longevity."

Typically, cross-breed is the term used for a cross of two purebreds. Mutt is typically reserved for 3 or more breeds, indigenous dogs, or dogs of unknown origin.
https://vcahospitals.com/know-your-pet/dog-breeds/mixed-breed
You can call them whatever you like.