I need help with my mastiff dog!?

I have a male mastiff, he is about 2 years old and a brilliant family pet, but when it comes to having him out on a walk he is a different dog he pulls on his lead he is aggresive to other animals and people, it has come to a point were i can only take him out at night when no one is around, i can let him off the lead and he will come back aslong as he has no distractions for animals or people :/ what do i do??