Texas didn't become a state until 1845, so the first federal census taken was 1850. "Republic of Texas: Poll Lists for 1846", compiled by Marion Day Mullins, shows one John McAdams and he is in Walker Co, TX.I suggest you look at land records for that county.Everton's Handybook shows records go back to 1846 in Walker Co, how lucky for you.When John Sr. dies his heirs should be signing off on final sale of his land to give clear title to the buyer, that should tie John Sr, and John Jr. together.
There is a book "The 1840 census of the Republic of Texas" ed by Gifford White.One of the TX public library with a good genealogy section, like Dallas or Fort Worth, probably have the book, maybe someone can check it for you.The FHL has filmed it, but there is a note not to be circulated to local FHCs.
If all else fails, you could request a photocopy of the page, pages with McAdams, there is a fee (I don't quote fees as they can change at any time.)
A quick look at the Family History Library Catalog shows they have filmed deed records for that county 1846-1886.You could order the index film out first to your local Family History Center, then you can see which roll/rolls of film of actual records you want to view.Those with "et al" are usually the good ones.First person named on a document is the one they use for the index, followed by et al, which means "and others."So if 6 heirs sign off, five will be the et als.Et ux means "and wife."