"You know the answer". son. Your baby brother is about to be born and the dog can increase chances of infection to the baby which will be risky.
'Dad',Why can't I have a dog?
'Hmmm...Maybe we can do something... I will think about this tomorrow. It is already past 10,you better sleep. Good night
'But,I real want one'. "There must be something that we can do".
Good night,'dad'
Zzzzzzzzzzzzzzzzzzzzzzzz
'Good Morning',dad. Have you thought the possible solutions for having a dog?
'Yes son, I have thought about some solutions... 1. Get a dog after we shift to our new house.2. Get a dog 2 months after your brother is born.3.Get a small pet 
Five Month Later....
Hmmmmm... I am not sure but I have a keep a promise so I will talk to everybody in the house and then tell you.
Okay Dad
Can we get a dog now? 'dad'. Noah has turned 2 years now and you said that we can get a dog after Noah is born.
Yes son, I have. So, I talked to everyone in the house and they said that we can, get a dog now. But, before you day anything I am only allowing you to get a dog because you have maintained your patience so I am getting you a dog! Here you go!
2 Hours Later...
'Dad, have you decided your final decision about getting a dog?