how should I write my first email to co-supervisor?
Two ideas only. I don't think you need to praise him in an initial note. Just mention the general area in AI that you want to explore. 

But the meeting should be at his convenience. Rather than tell him when you are available, let him set the meeting. If necessary you can indicate when you cannot meet if that is relevant. 

You can thank him and tell him you are looking forward to working with him when you actually meet.