Toilet training is really about forming a habit.
Your dog doesn't learn going outside is 'correct' and going inside is 'incorrect'. Instead, when they mostly ever go outside then that's the place they feel comfortable going. It's also the place they associated with the sweet reward that is the feeling of relief after peeing/pooing.
Much like we tend to pee on a toilet so, if you've ever had to pee in the wild, you'll know it feels slightly odd/strange to do so. That's what you're aiming for - for it to feel odd to wee inside and normal to wee out.
The reason I call it out is because when you understand this, you can see the way forward. Do whateve you can to ensure your puppy only ever has the experience of peeing/pooing where you want him to. Do that often enough to form a habit and you're there :).
At this age he has virtually no voluntary sphincter control. When he needs to pee/poo, he has hardly any warning himself and cannot hold it deliberately. So take him out every 30 mins or so, plus immediately after he has fed, played, slept, drunk plus whenever he starts to look like he might want a wee (he starts pacing or looking around).
You can treat him when he pees outside, but I honestly think it is a waste of time because the act of peeing (or pooing) is already inherently rewarding. The treat is surplus for most dogs.
A target word is a great addition, again it forms a habit that when he hears the sound "have a wee" then it feels good (relief) to wee.
Never scold or be upset if he has an accident indoors, you do not want him thinking that peeing/pooing leads to bad things - regardless of where he is.You risk him trying to do so when you are not around and this makes it all harder.
Clean any accidents up with a dedicated enzyme cleaner to prevent lingering smells that might confuse the dog into thinking inside really is a toilet (dogs are more likely to pee where they can smell pee).
Be patient. It will - most likely - take longer than you think. Voluntary sphincter control comes at around 12 weeks, but most dogs are 5-6 months old before they are reliably clean in the house and many have the odd accident up to a year old.