By cutting out 25 letters of the alphabet, you really reduce your choice. But luckily S is the first letter of loads of names for both girls and boys, so you shouldn't have a problem.
When my DC1 was born and I was back on the ward, I asked the woman in the next bed what her baby was going to be called. She said she and H had to take baby to see the priest who wd open the holy book at a random page. The first letter of the first word had to be used to select a name for the baby. She was dreading getting a 'bad' letter.
Hmmm, yes I imagine I will have to have that conversation if it ever becomes a real problem but until then I reckon I can humour him a bit. I have two already but if it happens this will be his first and only so seems fair I at least consider it.