I want to get access to a paper which hasn't been published by the scholar and I decide to write an email to him. What is a proper way to ask ?
My intention is to help myself formulate the scope of my next paper in the same field.
Just ask politely, and with the expectation that you may be reasonably refused.
Researchers vary greatly in their relationship to unpublished manuscripts. The spectrum ranges broadly, from some who are very happy to share anything they consider "not embarrassing," to others are extremely protective. Then there are many who will be just plain unresponsive for any number of reasons.
I would thus suggest that in your email, you say something along the lines of the following:
Importantly, remember that no researcher has a moral obligation to share work that is not ready for publication. The scientific community generally holds that work should be shared in a timely fashion only once it is reasonably ready for publication. You can argue about how to draw that line, but if a researcher really thinks their work isn't yet solid enough to disseminate or submit, then they're usually right. In that case, you probably wouldn't really want to build off of it yet anyway, since it is likely to change in the near future.
If your hope is to help yourself formulate your scope of your next paper in the field, you might not need to read their unpublished manuscript for this. I will assume that you've already read the other recent papers in the field, and the relevant review papers in the field, and this information you need really doesn't exist in published work.
That said, I'd reflect on if you really need to see their paper, of if a conversation with them could help you instead. Some scientists will find suddenly asking to read a manuscript off-putting and think you might be trying to scoop them of their follow-up paper, especially if you don't already have a relationship with them. Sending them an email (or even making a phone-call) asking if they might be interested in discussing their work with you over the phone or on Skype, might give you the opportunity to ask questions about the scope of the field, and help you build a professional relationship with that person.
Additionally, you can always suggest that as your first-choice, and follow up with "I understand that your time is very valuable..." and politely suggest that perhaps you could read their manuscript instead if they would prefer.