No he's not umbrous. Silver grey has dark ticking on the tips of the fur. Umbrous hamsters, for example a sable (which is Black eyed cream + umbrous) appears to be a black/dark brown hamster with cream coloured eye rings, but if you separate the fur, the colour at the bottom of the hairs will be cream.
The image below is a picture of two hamsters with the same 'base colour' which is black eyed cream. The one on top is a black eyed cream + umbrous, the one below is a black eyed cream.
Agouti colours, like your silver grey, has ticking which is a fine darkening of the tips of the fur.