If he's trained to go on the paper, have some paper outside for him too.
We start our puppies at 5 weeks with newspaper. And keep the bedding clean so they never have to sleep in their waste.
If you aren't already feeding on a set schedule, then do so. It helps keep the puppy regular too.
Our rule of thumb is 1/2 hour after each meal. We tell them 'Go potty."
Outside is not playtime. So, after they potty we praise them and bring them back inside. If they don't potty within say 10 minutes, we bring them back inside anyways and try again in 45 minutes.
Also, we give potty breaks during playtime, and at least every 2 hours when they are 8-12 weeks. After 16 weeks we lengthen it to every 4 hours.
And we always make a huge deal when they go outside - plenty of praise, maybe a treat, or a toy.
Accidents happen, and if I find them after the fact I just clean them up. If I catch the pup in the act, I just say NO! and take them outside quickly. If they finish outside I then praise, praise, praise. Spanking never helps, nor does rubbing their nose in it - then you just end up with 'sneak-a-leak'.
Some dogs get it faster than others. Consistency is key. We have a routine we follow daily.