Possibly a mix. I see some Basenji-like characteristics.
Basenji's are known for a square body shape - which is why the community will ask for (a side view) picture of the dog standing. We are looking to see if the length of the legs and back for that square.-like shape. Marvin's back is so much longer than his legs = no square shape, therefore not pure Basenji.
Another thing that would help would be the dogs weight and height. Again, we are trying to determine if the dog meets Basenji standards. But, from the picture, I'm taking a wild guess that he isn't in the "typical" 20-25# range. Another thing that he inherited by "the other breed". But you never know... there could be a lot of other combinations at play here. Marvin could be part (smooth coat) Portuguese Podengo, a breed that is similar to the Basenji.
As @Kembe mentioned, the only way to know for sure is with a DNA test. If you decide to get one, please share the results.