I definitely agree with everyone else who said not to take his water away, especially at his age.
You're definitely lucky you have a dog that likes to go outside in the rain. Many chis are the exact opposite and would rather find a place under the sofa or bed to pee, instead of going outside if it's wet or cold out. I would encourage it instead of discourage it if I were you.
Unfortunately, at 8 weeks old, he definitely doesn't have too much control over his bladder. It's just something you have to deal with. Take him out as much as possible, give him treats/praise when he uses the bathroom outside, and clean up after him if he does it inside. It will also help him to learn a command to relieve himself, such as "go potty" or "hurry up." Then, after he's all done, play with him out there for a few minutes, so he doesn't think that finishing his business means the end of fun (which may cause him to stay out there for as long as possible and take a long time to finish what he has to do.)