He should read the P46 questions and decide which box to tick. It is not your job to guess what box he would tick when presented with a P46. You don't know everything about his circumstances, and the P46 is a legal declaration by HIM, that HE signs.
I can't comment on adding a director to Sage. It should be very similar to adding a normal employee, but there are additional settings.
If the director is planning to take wages then you need to set him up on payroll.
Ask him to fill P46 and use this data while keying in details on your software.
In addition to this, whike keying in his details on the software, you need to set him up as a director. You will have an option for this on your software. What this does is defer the NI payments until the NI threshold.
Hope this helps