Hi,
I just went through something similar so I hope I can be of help. I have a one year old male Chihuahua who had always been an only dog and never had to share toys or attention. On weekends, we would go to my mom's house and he always loved playing with her dogs (a male and a female). He never had issues with either, but they are older dogs and my Chi, being a puppy, always wanted to play... nonstop. A few weeks ago, I took him over to a humane society hoping to adopt a cute female Chihuahua I had seen online. Once I got there, she had already been adopted but we spotted a very cute male Chi and decided they should meet. I had stocked up on my reading/research before every adopting a puppy... which I suggest you do. At first, I was worried because my Chi didn't pay any attention to him but the shelter workers told me this was fine at first as long as there were no signs of aggression.
I would suggest they meet in neutral territory. What I mean is... someplace that neither dog is familiar with, especially since they are males. For example, if they first time they meet is your home, your lab may feel another dog is in "his" territory, and may feel the need to defend it. So, ask a friend or family member to help and take each dog to a nearby park (one neither dog has ever been to) and have them meet there. The first few days will be the hardest and I suggest you keep an eye on them. On a sidenote, it will help so much if both dogs are neutered. They won't (or shouldn't) feel the need to mark any territory, which will make things extra easy for you.
After they met, they shared a car ride home and did extremely well together. Once we were home, I kept an eye on them and my Chihuahua continued to ignore that there was another dog in the house. At first, he was possesive of his toys and would not let the "new dog" touch them. He showed no signs of aggression though, so I let it slip. After all, this was a big step for him. The second day was the worst. While my Chi was napping, the new puppy decided to hop in his bed and take my Chi's favorite toy. My Chi growled and attacked him. I put my foot down and gave him a good scolding and sent him to his crate. After that incident, they've become best friends.
I believe that is because we did a really good job training my Chihuahua and he is a truly obedient dog. In his eyes, I am the pack leader and what I say goes.
Dogs, like wolves, are pack animals and they need to establish a chain in command. This is how they see it: Me, adult Chihuahua, puppy... in that order. Let them sniff each other and figure out who's going to be second in command. Most likely, it will be your lab and the new puppy will be okay with third place... unless you have an extremely laid back lab. It all depends on personalities. A very important thing to remember... I know the puppy is going to be cute and impossible to ignore, but always, always, always praise your lab. Don't make him feel like he's second-best just because a new dog came around. You need to show him in every way possible you love him and that having a new friend is a good thing... not a war for attention. I wish you a lot of luck and it is possible, trust me! Now my Chi's love each other, they play together, sleep together, go in the same crate together, share toys, attention and sometimes even food. It's the best decision I've ever made and we all love them both. Good luck to you! I am going to give you the links to websites that really helped me.