Lewis is the best spelling to choose if you want this pronounced with the S. Lewis has been in the Top 5 in Scotland since 2000, and is one that parents in the U.S. are just beginning to rethink.
Lewis was a Top 100 name in this country from 1880 to 1930, reaching Number 30 in 1880. One of its most famous namesakes is Lewis Carroll, born Charles. A current bearer is comedian Lewis Black. Lewis is also a very common surname, as in Lewis & Clark, Jerry Lewis, Jerry Lee Lewis, Daniel Day-Lewis, Sinclair Lewis, Leona Lewis, and many other notables. Interesting fact: Louis Armstrong pronounced his first name as Lewis.
Lewis is currently Number 131 on Nameberry, while Louis, the choice of Sandra Bullock for her son, is 109.
Nickname/middle name Lou is making a comeback for girls (as in Keri Russell's Willa Lou), so why not Lew for boys?