why does my 4 month old bulldog hate going outdoors?

[Samantha]

Hello, we have had our bulldog puppy since he was 15 weeks old and he is now just over 4 months old. He is afraid to go outside, doesn't matter whether its the garden or park or outside the house, he cowers low to the ground and sometimes just stops and refuses to move - he will not eat treats when he is like this. When we get the lead out he runs and hides and looks scared - we have tried to make it really positive for him and he has never been scared whilst with us so we are unsure why he is like this. We put his lead on him in the house and offer him treats which he wont take, he's not interested in his toys either. We took him to a puppy class the trainers were not interested in us as our puppy wouldn't join in with anything as was shaking like a leaf! We cant train him to toilet outside as he will hold it in - sometimes until the following day!
He also cowers, barks and growls at people coming to our house - we had our friends round and it took him 4-5 hours to come and sit near them - he seems petrified of everything, we try to get people round all the time to get him used to strangers etc..
Can anyone provide some helpful suggestions?


thanks
thank you for your comments so far. I am very positive with him when we do get out of the house but he will refuse to leave the house of his own accord.
Im not sure what was meant by "internet training" as i am just trying to get some advice from other dog owners.
Training within the house goes fine, he sits, lays, waits and come and he is extremely playful but out comes the lead and thats it - not interested!