In most ways, he’s obviously a lab. But he’s 98 pounds, but not fat, which is big for a lab. And his ears are very low-set on his head. And his muzzle, rather than being a rectangle, is pointy, with a downward curve towards his nose. Plus, he has a lot of loose skin on his neck.
I suspect, therefore, that Sonnyboy has a coonhound grandparent.