What breed is my dog?

We adopted a male dog from a shelter and for the life of me I don't know what breeds he might be. The shelter said "lab mix" but he is definitely NOT. He has lots of energy but is content to sit and chew his rawhide bones. He has a slightly curved tail, and markings like a Rottweiler but they are gray. He has longer hair, but not really long... I haven't been able to get a good picture of him yet, and if no one can get a good picture of him from this description, I'll try to get a picture and ask again later. If you want to ask questions about something that I missed, feel free, I'll just add them. Thank you!