The thing is, he could be a pure bred basset hound folks,even with the longer legs and shorter ears.When bassets are young only someone who knows what they are really looking for can see the difference,such as ,the ears should be set below the eyes and look as if they are coming off the neck when in repose(relaxed).Legs could be a little more difficult just because puppies are puppies and they all look short at that age. Do you have any registration papers for him. That doesn't mean he isn't mixed but it is possible his reg.parents look just like him. It all comes down to his breeder's qualities,the people who owned the parents.
My little Princess Bella, is a Bagle Hound, please see my avatar, but she is just like a small Basset with Beagle colouring, she only weighs in at 15 kilos but she's only a year old so she still might grow a bit.
Oh he is really adorable. I think he is most certainly a beagle. Crossed with some basset in him. Based how how his ears are set, and his head. But the legs long as they are..the actual feet resemble a basset. His whole body is long like a basset. But the ears are shorter like a beagle. So I think most definitely his is a beagle basset mix. You have the best of both worlds there.