AideenMongan's list "S Boys" of 3 great name ideas: Sorrell - Septimus!

S Boys

Soft, amber-hued herbal and autumnal name that's used most often to describe the color of a horse. The proper spelling: Sorrel.

Sorrell

Seanan might be considered an Anglicized form of Seanán, itself a modern form of Senán, an Old Irish name meaning "little old one", giving it a wise and ancient feel. The name of a 6th century saint and monk, Seanan might also be considered an elaboration or a diminutive of Sean, the Irish variation of John. By combining it with the -an suffix (meaning "descendent of" or more literally "little") it could be a way to freshen up the now well-travelled Irish name. 

Seanan

Septimus is one of the more dashing of the birth-order Latin number names that were revived by the Victorians. So even if you don't anticipate son number 7, you might be bold enough to consider this relic, certainly preferable to sixth-son name Sextus.
 Septimus was popularized by the Roman Emperor Septimus Severus, a patron of arts and letters. There was a Septimus in Wilkie Collins's The Moonstone, in Dickens's The Mystery of Edwin Drood, and in Trollope's Barchester novels. Probably it is most familiar to modern readers as one of the principal characters in Virginia Woolf's Mrs. Dalloway, and as Harry Potter wizard, Septimus Weasley. There is also Septimus Heap, a series of bestselling fantasy novels by Angie Sage.