Ask for money, get advice. Ask for advice and get money
You should go to him, tell him your idea and ask him if he can see any problems with it.
In the best case, he'll get really excited about the idea and ask if he can be involved. In the worst case he won't show any interest (which means he probably wouldn't have invested anyway).
(This assumes that he's not going to try and steal your idea and do it himself. You'll also need to make sure he understands it's your idea - not something you've been working on at work.)
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There's a saying:
Ask for money, get advice. Ask for advice and get money
You should go to him, tell him your idea and ask him if he can see any problems with it.
In the best case, he'll get really excited about the idea and ask if he can be involved. In the worst case he won't show any interest (which means he probably wouldn't have invested anyway).
(This assumes that he's not going to try and steal your idea and do it himself. You'll also need to make sure he understands it's your idea - not something you've been working on at work.)