Taking a dog off leash is something you need to be VERY careful about and have a very good reason to do it. If the surrondings have stray dogs or possible motor vehicle movement, then its a BIG NO NO. I say this because I am still regretting the 5 seconds my little boxer had off the leash. A speeding car ran over her face and she is still battling the complications. It is NOT WORTH IT!
Having said that, there are 2 things you need to take care of:
1. Socialization: As Dinakar rightly points out, gradually expose your boxer to several different animals and people. Start at a distance and very gradually reduce distance. Socialization has to continue all through a dogs life. Actively seek out different situations you want to get your dog used to, and keep giving him controlled exposure
2. Loose leash walking: While walking him, if he pulls for ANY reason, Become a Tree. This means, stand still as a tree. Don't move anywhere. If you move, your dog will learn that he can pull you in the direction he wants to go. If you refuse to move, your dog will learn that when he pulls, walking stops. Just wait patiently till he either sits or reduces the tension on the leash. Then move forward. Initially, you might only be able to walk 2 steps before he pulls again. Don't lose hope. Just become a tree again. On your first walk, you might only manage to walk 200 meters or less. But gradually your dog will learn "oh! when I pull my walk stops. i should take it easy".
Boxers are energetic wonderful dogs that have a really short attention span and have play on their mind all the time. It might take you a while to teach him not to pull. But just laugh at his goofieness and his playful behaviour. This will help you to have patience, while he slowly learns his lesson. Once he does, he will be such a wonderful mate. Boxers are awesome!