I wouldn't think about adding another puppy until my current dog was an adult and consistently solid in training and general life/house skills.
OP, I know it appears as if your current dog is settled enough to add a puppy sooner, rather than later. But your guy is still very much a puppy himself and will continue to be for another year or two (at least). There are so many shifts made in the first 18-24 months. Sometimes the dog you have at 9 months is nothing like the dog you have at 2 years, or 3 years. Give your current guy the time he needs to show you who he is and what he needs.
Your current dog deserves your best effort, which includes as much work as you can possibly put in as far as training, socializing and exercise goes. Getting a second puppy before your current dog is done with requiring so much of you as an owner is short changing him. And it would be short changing the younger puppy as well, because that puppy in turn deserves the same kind of attention and effort.
Love your current dog, train and socialize and make him the best dog that he can be. And then add a puppy. You won't ever regret being patient and waiting. But you sure could end up regretting bringing a second puppy into your home too early.