Your caution is well received. At a large park we take him to we use the strict rule that if another dog approaches, on a leash, we pick him up and ask the owner if their dog's vacinnations are current and if he/she is gentile. If so, we let them greet and play. If I see a stray or dog not with owner, I don't even approach nor let him approach. Our pup just had his 2nd series of shots but still isn't "cleared" till the 4th, which I understand. Our vet agreed that it is better to get him exposure to the environment and be responsible than to keep him in house for the 14 week or until his series is finished. He put it to me this way (paraphrasing), "I've had to deal 10 times to 1 more cases of dogs with horrible to down-right threatening socialaiztion issues than Parvo. Of course, parvo kills, but I've also had to put down a dozen dogs just last year due to their inability to function socially (biting children, etc). It's worth the risk and the risk is very small if you are careful and smart about his interaction with other dogs and droppings left by other dogs. I'd rather have you take that chance than to never let him get this exposure till later in his youth".