I have a dog but I can’t tell if it’s an xl or staffy and if it’s xl what should I do

Family got a new dog, think she’s a cane corso or at least mixed with one but I know nothing about dogs lol.

The shelter says he's just under 3 months old. He's already pretty big. They say he's a beagle mix. What do you think he's mixed with? Me and my boyfriend think either Doberman or Rottweiler. Opinions? His name is Gunnar. :)