How can I email him and ask him these things and ask him to consider me for a position?
I'm not sure that I'd try to put this in an email at all. This is the kind of thing that you'd want to ask in a phone call or in-person.
I take it that you have an email address but not a phone number. So I might say something like
Hi [his name],
I heard a rumor that you had started a new company. That sounds interesting and I'd like to hear more. Please share.
Thanks,
[Your name]
Unless he blows you off entirely, he'll tell you at least the name of the new business. That should be enough to get the phone number if he doesn't give you that.
If you have any career questions, you might also ask them here, e.g.
I've been thinking of moving from developer to project manager. Do you have any suggestions?
You also might share an anecdote with him. If your old conversations included things like "And did you hear what Jack did? He ..." then it might be appropriate to include something similar. Or to tell him something that you did at work, assuming it is not confidential for some reason.
You could also offer to buy him lunch to catch up. You offer to buy so that he doesn't feel like you're welshing food from him. If he wants to buy instead, that's fine. But the idea is to suggest that you value talking to him enough to pay for his meal. Realize that this may actually involve you paying for the meal. Perhaps he's looking for proof. Or maybe he's cheap and likes the idea of free food. Either way, you get your in-person meeting for a relatively nominal cost.
You may want to suggest a location so that you can control the cost somewhat. Unless you know he loves fast food, I'd look for a decent sit-down restaurant. If he counters with something too expensive, you can tell him so, "That's too rich for me."
In any case, either a phone call or an in-person meeting (like lunch) would allow you to say something like
I really enjoyed working for you last time. Do you think there's a place in your new company for me?
I would suggest asking questions about the company first, e.g. what it does, etc. That makes it seem like you are interested in the company and not just interested in leaving your current job. If he asks why you are interested, you should have an answer ready.
Part of the reason to ask on the phone or preferably in person is that you should get immediate feedback on how he feels about it. Is he enthusiastic? Reluctant? Unsure? He can simply not reply to an email. If you're already in the midst of a conversation, it's hard to just hang up the phone or leave lunch. He'll pretty much have to say something. And even a firm no is useful information for your career planning.
He also may offer unprompted in response to the email or in the conversation later. But if he doesn't, I see no problem with explicitly asking. Then you'll get an explicit response and not be left hanging.
As people have suggested in the comments, not explicitly asking avoids the danger that he will tell your current employer that you are unhappy. As they noted, that may have career implications that would be better explored on The Workplace.SE.
This is my advice in terms of dealing with him if you aren't worried about issues with your current employer. The idea being that he should still think well of you at the end of your interaction whether he hires you or not.
