Welcome to Pet Talk! Your pup is adorable. He reminds me some of a flat-coated retriever, but really it's so hard to tell when they are young. When I got my girl Tasha, I was told at the humane society her mother was a German shepherd/greyhound mix, but I was somewhat doubtful and she looked kinda lab/GSD at the time.

Here she was at eight weeks:




12ish weeks:




7 months!


And present day (7 years old)



So I'd say it'll be kinda tough to really guess for sure until he matures. But My golden retriever Tommy was 65 pounds at 6 months old, so I think he'd weigh more at 5 months if he had newf in him, although if he was newf and a more medium breed he may not be.. you just never know!