Smart of you to pay attention. Watch out for continuing coughing or wheezing noises.
Now is also a good time to start working on his leash manners. Just because he's a toy doesn't mean it's ok for him to lean into his harness like a plow horse dragging a harrow. The front clip harness will help, but it's good not to be dependent on it.
As you are walking, monitor the slack in the leash. There should always be a dip in the leash. If the leash goes completely straight, Jaanu is pulling too much. Stop walking. Simply stand there. Jaanu will probably continue leaning into the leash, attempting to drag you over to whatever caught his attention. He may even start bouncing up and down in frustration. Continue standing there.
Eventually Jaanu will look over his shoulder with an expression of, "Come on, man, what's the hold up?" That shift in weight and body will give you a little slack in the leash. Also, now his attention is back on you. Reward him by taking a step forward. He will probably go right back to trying to drag you along. Simply stop and wait it out again. . You will feel like an utter fool for the first few sessions. Your next door neighbor may wonder why you are standing 30 feet from your driveway staring down at a frustrated puppy. Be patient. The time you invest now will help you and Jaanu later.