Have him "speak" to go out. In other words, stand by the door and have him bark before you open it. He will learn to let you guys know when he needs to go out. Before we did this, our dog would go to the back door and wait for us, and if nobody realized he was back there, he would end up getting impatient and peeing on the floor. He learned really quick that he had to bark before the door would open... Although, getting him to do it the first few times involved me making little puppy barking noises. The best time to train this is first thing in the morning, when he really needs to relieve himself and he really wants to get outside.
Once he speaks and you go out (and it is good that you have him on a leash) then take him to wherever you want him to relieve himself and tell him 'do your business' or 'make it snappy'. Any little phrase that you can repeat to make it known that he is out their for the sole purpose of relieving himself. This helps later on when you need to go out somewhere and you want him to pee before you leave. My dog is about 2 years old, and I can take him outside and tell him to do his business, and he will go even if he doesn't really need to.
After he is done with that, you can give him a treat and praise him, then take him inside. When he is housebroken a little better, you can reward him with playtime outside when he is done, but for now, keep bathroom trips and playtime trips seperate. Take him outside about 15 minutes after every meal, and maybe once every hour or so (depending on your dog) specifically for going to the bathroom. Also, activity stimulates the dog's bowels, so if anyone is playing with him really actively (like running and chasing) then take him out for a bathroom break.
That's all I can really think of at the moment. Mostly, it's just a matter of patience. Even the best trained dog won't be completely reliable for a good 4 or 5 months. Accidents happen, so expect to be cleaning up for a while. On the bright side, if your dog is only about 8 weeks old, then his bladder control isn't anywhere near what it will be soon. His control should be 10x better in just a matter of weeks.