Maryjo@mjgen.com states on Ancestry.com family trees, that William was the son of George May and Elizabeth Rhea; that he married Nancy J. Wilcox on 21 September 1843 in Mason County, Illinois, and that he died about 6 months after his marriage in 1844. But your William A. seems to fit perfectly for the son of George and Elizabeth, as I think I have found Nancy J. Wilcox's parents, living in Macon County, Missouri in 1850. Henry D. Wilcox and his wife, Sarah, were in Cass County, Illinois in 1840; George May and his wife, Elizabeth Rhea, were in Menard County, Illinois in 1840 and these two counties adjoin Mason County, where William married Nancy Wilcox. Elizabeth Rhea May reportedly died abt. 1841 and George May subsequently married her sister, Mahala [a widow] and Catherine Coen, before moving to Missouri. You might want to contact Mary Jo to share data.
1850 Macon County, MissouriDistrict 52page 138 #277 #28419 September 1850 William Mayage 25minister, M.B.b. KY Nancy J. May22IL Sarah E. May4MO Lucinda May2MO Ephraim E. Arterbury 1MO next door: Arterbury, Henry T. 20farmerKY Margaret 18IL James1MO Appears that James and the above Ephraim are twins? Perhaps Nancy was helping out? page 127#125 #13022 August 1850 Wilcox, Henry D.46farmerKY Sarah40KY
1850 Bogle Twp., Gentry County, Missouripage 225 30 October 1850#434 #434 May, George55bricklayerVA Katherine 25IND George17 farmerIL Nero10IL Nevi10IL [the twins are stepsons of George; believe the surname is Pulsifer] Sarah Ann4MO Thomas2MO Hugh2/12MO
1840 Cass County, Illinoispage 39 Wilcox, Henry1 male 30-40Henry D. aged 36 1 female 10-15Nancy J. aged 12 1 female 30-40Sarah aged 30
1840 Menard County, Illinoispage 312 May, George1 male 5-10George Jr.7 2 males 15-20JamesWilliam 15-17 [Mary Jo shows the above two as both born ca 1820] 1 male 40-50George May 45 1 male 50-60 1 female under 5Mary Ann 5 1 female 10-15Nancy 15 1 female 20-30Eliza 22 1 female 30-40 Elizabeth Rhea May 38 Kids' names were taken from Mary Jo's entry; see above link.
There is also a probate file for George May Sr. in Macon County, Missouri. He had the same administrator as William A. May: William A. Wilks. The file numbers for George May, Nancy May and William A. May, are all close in number, indicating that they were filed in close succession of one another. You may get extra clues by asking for lookups at the Macon County, Missouri GenWeb site, where I found these probate listings in their index. There is also an obit referenced for William A. Wilks: http://www.rootsweb.com/~momacon/http://www.rootsweb.com/~momacon/