You've made great strides in pooch's training, so let's test his knowledge in the great outdoors. After all, as well as he may listen to you inside the house without distractions, how will he fare with birds, grass, and other people around? Once your pup has mastered commands and you've varied the amount of treats, begin seeing how quickly he'll sit without you saying anything or if he can hold his sit for a couple of seconds before a reward. It may sound difficult, but when you've taught your leashed pup to sit for his food, he'll begin doing it as soon as you grab that dish all on his own!
See if you can get your dog to sit and stay as you back away while holding his leash in a controlled area outdoors, like your deck or backyard. If you're asking him to hold a sit, give him a treat before he's supposed to get up. When you're ready to move on, say the release command you've chosen — that way he knows it's time to stop performing that task. If you lose his attention, try again with fewer diversions. When he's ready, take it outdoors in a busier environment like a park.
It's hard for most pups not to get a tad distracted in the beginning, and don't give up if it doesn't happen right away. Try to work this practice into your daily walks or jaunts to the dog park — it'll take time, but the promise of having a well-trained pooch is well worth that investment!