What Breed Is My Dog?

Can anyone tell me what breed my dog Dexter is? We got him at a shelter 3 weeks ago. He is now about 4.5 months old and weighs about 17 pounds. They said he was a Labrador Retriever mix from Puerto Rico. My wife and I are not sure if he is actually is Labrador Retriever and that's perfectly fine, he's adorable and fun loving. He doesn't bark, but whimpers a little, he's very tame and a very quick learner.

We are crate training him right now so when it's time to come out of the crate, we sit on the floor and open the door and he comes out and walks right on to our lap and gives us hugs and kisses and just hangs out on our lap until we take him off to put his leash on. We took him to the dog park and he was the fastest dog there, very quick and very good agility. I hope this helps and thank you all so much for your help!