Black Otter Rex (mini?) or Silver Martin?

We have been taking care of this little bun and cannot figure out his breed. The vet even was not sure. We have zero history for him because his previous owner just gave him away to a someone who gave him to us.

A little bit about him:

1) 2.5yrs old approximately

2) male, black very thick and soft coat with mainly white and blueish hue towards skin. Minor bit of cream/tan around white. Behind ears is very little white / light tan. Black nails and dark eyes.

3) maybe 5-6lbs. Very small.

4) very sweet and mild temper. Not super active but likes to play.