
You should put him on his lead to take him out and then reward him when he goes. I use the word "busy" when he performs and now he has got to the point where I can ask him to go and do a busy and he will trot out and do his utmost to have a pee.
If he had dire rear he probably got used to going indoors and you will now have to go back to basics with him.
He may have been frightened of something out in the garden so perhaps you could try and make the garden seem like a fun place to be. If he is toy orientated try playing out there for a few minutes at a time. If he is food orientated ( definitely a favourite with mine :D) try scattering a few treats on the ground for him to find.
My dog was quite badly frightened by a firework in the summer and did not like being out in the garden after this so I started giving him his breakfast in the garden to try and get him to associate it with something good and this has worked for him.
Someone more experienced will probably be able to offer you some ideas as well.