Is he scared of the dog? If not, hes just bonding...LOL, as crazy as that seems, but your baby thinks the dog can hear him because hes trying to get a certain reaction from the dog to no avail so he keeps screaming. Teach your baby to give the dog a treat to get its attention instead of screaming. That may work..
Is it a happy excited scream? Or fear. Like the PP, if it's a happy scream don't worry about it. I baby sit my friends daughter almost every day while she works since she is single mom. I have a dog and cat and her 14 month old screams when she sees them and has when she was about 5 to 6 months lol. They are used to it and no my dog gets excited with her and lays down for her to pet him.
Just let the baby get used to the dog. As long as s/he is not afraid of the dog, just show him/her how to pet the dog gently and say "shh, pet quietly!" I did this to get my son used to being around our cat. I know cats are different than dogs but my cat is HUGE and as long as the animal is not aggressive, they will get used to the baby and even start protecting him/her like its own.