Need help for a troubled dog!!! Please help!!!?

We have a dog (Freddy) who is two years old. he is Labrador cross Mastiff. My mum bought him from this lady. Ma mum says she thinks Freddy was miss-treated and wasn't fed properly. He is sooo adorable and is so friendly to us, but he has so many problems. We brought him up fine and got advice on how to train him from a professional trainer. But he has the most terrible fear of men. Any man. weather he is on the other side of the road or if we are driving past a man he will balk at him with his tail between his legs and go mental!! he will even balk at some lady's to. He is fine when he is of the lead and will play with other dog nicely but as soon as he is on the lead again he will balk and growl at any dog that comes near him. there is 1 man that he is not afraid of and that is my mums boyfriend who lives with us. if you have any advise please help