You say you only have initials for Samuel's father, but you didn't give the initials.Is he the one you say was born in TN in 1820?
I do not find a Thomason on 1860 census in Lamar Co, TX, but there are Thompsons.
1860 census Lamar Co, TX:
Thompson, Wm. A., 45, b TN
" Elizabeth, 45, b TN
" Priscilla, 25, b TN
" Wm. ?., 23, b TN
" Martha, 21, b TX(sic)
" Thomas ?, 20, b TN
" Ina L., 18, b TN
" Jos. N., 16, b TN
" Jms W. E., 14, b TN
" ____ G., 11, b TN (male)
" Ann E., 9, b TN
" Amanda, B., 5, b TN
" Sarah D.J., 3, b TN
For Wm. age 23, can't be sure if initial is A., but father is Wm. A., so good likely hood it is his too, giving you a W. A.And, the Joseph N., could be your J.N.
Or, on the 1870 census there is another "possible."
1870 census Lamar Co, TX:
Thomason, B.W., 60, b TN
" N.L., 52, b TN
" M.W., 29, b TN
" O.L., 24, b AR
" L.M., 21, b AR
" A.C., 20, b AR
" H.H., 18, b AR
" J.N., 13, b AR
" S.D., 10, b AR
" W.A., 1, b TX
If S.D. is Samuel notice brothers on each side of him are J.N. and W.A.What is your source for birth year of 1820 in TN for his father?And, what is your source that Samuel was born in Lamar Co, TX?It is not unusual for people to say they were born in a certain location, when actually they grew up there, but were born elsewhere.
1880 census Fannin Co, TX, looks to be the same family as above, and the S.D. is a Samuel:
Category: Name;Relationship tohead of household; Marital Status; Gender; Race; Age; Birthplace; Occupation; Father's Birthplace; Mother's Birthplace.
B. W. THOMASONSelfMMaleW70TNFarmerNCNC
Nancy THOMASONWifeMFemaleW62TNKeeping HouseNCNC
James THOMASONSonSMaleW22ARWorks On F.TNTN
Samuel THOMASONSonSMaleW20ARWorks On F.TNTN
William THOMASONSonSMaleW11TXWorks On F.TNTN
Minnie WAGGONEROtherSFemaleW11TXAt SchoolTNTN
Julius NORTONOtherSMaleW20ALWorks On F.ALAL
James CREEGSOtherSMaleW20MIWorks On F.MIMI
Happy Hunting (^_^)