One thing that I know is that cheating almost never ends well. I agree with above that it's better to tell them and let things work out the way they will, rather than lying to both of them.
I can't tell... is V your ex-fiance, or a third man that you met after you broke-up with your fiance but before you started going out with S?
If V is the ex-fiance, I would cut him out of the picture. My mom has anxiety and BP and the last thing I would want for her, or for you, is a partner who is oppressive, possessive and jealous.
If V is someone that you met after your fiance, and started seeing before you met S, it doesn't sound like he's on board with the polyamory, since he already made you choose once, and after only one week.
But if you believe that polyamory is something you want to pursue in your life, you will need to tell S at some point, so sooner is probably better than later. If you try to push down those feelings all your life, it will cause you pain.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker