He's definitely a cutie, and I agree that there is likely to be some lab or other type of retriever in the mix.
With my retrievers, usually the first tooth falls out almost exactly at four months.
Can you give us a clue about his weight?
As he grows, then it may be easier to tell what else is in his lineage, but always it is a guess. Read the blue dot discussion about photos that help us identify possible breeds in the mix.