since the mom was a mix then a lot can throw off different looks. Do you know what his siblings looked like?
He does look, lab-pit mixish. Could have boxer too. if he has any shepherd I would say a very small part. I have seen adult mutts like him around these parts. They are all great dogs.
He looks like he has some mastiff in him maybe great dane and his eyes look like Norwegian ridgeback?
How old is he now and how heavy?
Beautiful boy love the eyes.
The clinic I work for has just started doing blood tests that can identify most of the breeds your dog has in it.