You should always accompany your puppy outside for potty breaks. You’re there not only to ensure that he actually goes, you’re also there to reward your puppy with a treat for going in the proper spot. Wait until your puppy finishes eliminating and immediately give him a tasty reward for a job well done. If you wait until you get back in the house, your puppy won’t make the connection between his elimination and the reward.
Consistency is crucial. This may mean taking time off work to be there to take the pup outside every 20-30 minutes when they are awake. Crate training will also help the 'penny drop' as it teaches the pup to hold on until taken outside. Then, be sure to stay with the pup in the yard so you are there to reinforce how clever they are when they do go in the right place.
Many owners appear disappointed that their young puppy will not toilet when out on a walk, yet relieves itself the second it gets back home. This is because the puppy has been taught to toilet only at home (hopefully in its garden), and being creatures of habit, they often wait until they have returned home before evacuating their bladder and/ or bowels.