Ok so I read over... this question and there were some really good tips here, however... My 3 month old Shetland Sheepdog just won't go outside in the first place...
He is paper trained for inside, and in the day he has someone come see him to look after him, to let him out in the garden etc. Our aim is to give him treats when he toilets outside - at this point I'm not fussed about where in my garden he toilets so long as it IS in my garden, however he has stopped toileting outside and waits to go inside again. He used to toilet outside, and would receive plenty of fuss etc for doing so. He has a daily 20 minute walk - with a good 20 minute break in the middle where we sit and he can sniff around and relax - but he won't toilet on walks either, even when we walk with a friend and her dog goes.
Recently we spent 4 hours out with a friend, in her garden, on a walk etc and he held his bladder for the entire time and only went when we got home, which seems an incredible feat for a 3 month old small breed pup...
What we are doing... is that when he starts to go to the toilet inside we open the back door and move him to some newspaper outside (this is linked to how my in laws toilet trained their Shelties) however he then just clams up and won't continue... Until he is allowed back inside. Often he will toilet inside and then sit at the back door and ask to go out, which is the wrong order!
What can we do to improve this situation? Will moving him outside eventually click, if we use treat training, and as he becomes more controling of his ablutions?
Update taking Cedrics advice we played ball in the garden for a good 10 minutes and got our first outdoor wee wee in three weeks. Duely rewarded too!