You might be on to something there. That would mean he would have been born at sunset, as one's rising sign is like a clock, with your own sign beginning at sunrise; making your opposite sign at sunset.
This brings up the discussion of rising signs, and those who have approached me who wanted their charts done by hand after getting a most inaccurate reading from computer programs/websites. Many astrologers use a complex method to "calculate" the rising signs at different times depending on Daylight Savings, when a sun sets/rises on a particular day, etc. This has always been a complex and horribly inaccurate method, to my way of thinking. I have always used the chart that I originally got from the books of Sydney Omarr (one of the greatest astrologers in the business, quite possibly of all time), which I now know has been referred to by other names, such as a "harvest cycle chart". This method is far easier, and way more accurate.
How it works with rising signs is that they are supposed to make a complete circle around the zodiac in the course of 24 hours, so the harvest chart evenly divides them every 2 hours, regardless of time zone or whether or not it's Daylight Savings (because not all states/regions recognize Daylight Savings time). With the other methods that astrologers use, sometimes the rising signs do not make a complete transition in a day's time, so this is another reason I find those readings to be inconsistent, and downright confusing to those who are new to astrology.
I say this because I have had some approach me with readings and they will tell me, "that's not the rising sign I was given on the computer!" Well, I use a much more old-fashioned, but still very reliable method. These same people will also tell me upon getting my reading that my assessment was more accurate as well. Probably because I found the correct ascendant! (I say "probably", because I'm not taking credit for it; it was all there, written in the stars all along!)
Just say you want to make a star chart for him and ask for the time he was born. And thank you for taking the time to make my chart C. 
