I'd guess JRT just because in the last pic it looks like he has short legs, which many JRTs have, and Rat Terriers have long legs and tend to be lean. JRTs can also have the upright ears too.
Some JRTs:
Rat Terriers:
Otherwise, the breeds look so similar it would be hard to figure out which one was in a mix.
Some JRTs:



Rat Terriers:



Otherwise, the breeds look so similar it would be hard to figure out which one was in a mix.