I agree the link between Thomas Walling and Ralph Wallen should be marked uncertain, but I think the lack of repetition of his parents names in the next generations may be explained by the fact that Thomas himself was a black sheep and probably estranged from his parents early in his life. Ralph Wallen appears to have died when Thomas was a boy, and Thomas is listed among the first settlers who settled Rhode Island along with Roger Williams in 1636, although he would still only have been a boy at that time. (See Farmer, "First Settlers of Rhode Island." In The New England Historical and Genealogical Register, vol. 1:4 (Oct. 1847), p. 291.; image, Internet Archive, https://archive.org/details/newenglandhisto56unkngoog/page/n308/mode/2up). Thomas is the only Walling that Farmer lists among the first Rhode Island settlers, so if he is correct then Thomas was probably apprenticed to one of the other initial settlers of Rhode Island as a boy, possibly after his father after died in Plymouth. He then appears in the records from time to time after that getting in trouble because of womanizing -- e.g., in 1650, when he and another man are apprehended in Barnstable for running off with the wives of two other men:
The fourth of Aprell, 1650. Tho Wallen, Richard Carle, Gorg Way, Katheren Warner, and Mary Mills were apprehended at Barnstable, in the jurisdiction of New Plym; and on the eight day of Aprell, aforsaid, they being examined before William Bradford, gent, Gouer, Willam Collyar, and Willam Thomas, gent, Assistants, confessed yt they, the said Tho Wallen, Richard Carle, & Gorge Way did healpe away Katheren Warner & Mary Mills, who were run away from theire husbands; and for yt purpose yt Richard Carle aforsaid did steale his fathers boat, which they came away in; it was therefore ordered by the Gouer & Assistants aboue mensioned, taht the aforsaid Gorg Way, Katheren Warner, & Mary Mills should bee sent from constabel to constable to the place from whence they came, wh is a place called Winter Harbor, near Richmans Iland to the eastward; and yt Tho Wallen & Richard Carle aforsaid bee comitted to ward; all which accordingly was forthwith formed.
(Neuzil, "Women in Plymouth Colony," The Plymouth Colony Archive Project, Fall 1988 (http://www.histarch.illinois.edu/plymouth/PCR.htm)
After this incident, there is often-cited letter from Roger Williams commenting on Thomas's sketchy past when his planned marriage to Mary Abbott was announced, in which Roger expressed his hope for Mary's sake that Thomas would "forsake his former courses." (https://archive.org/details/genealogicaldict00aust/page/n445/mode/2up)
But that apparently did not happen, as he later appears in the Providence records again having abandoned Mary and their children in 1663. (See Rogers et al., Early Records of the Town of Providence Vol. III (Providence: Snow & Farnham, 1893), 32-33; images, Hathitrust.org, (https://hdl.handle.net/2027/coo1.ark:/13960/t7xk8ws6z)
On whether Thomas was actually the son of Ralph and Joyce, I have not seen any direct evidence documenting the connection. Most researchers who assert the connection seem to base it on the fact that Thomas appears to have been born in Plymouth or the Bay Colony and to have come from there as a boy without his parents to Rhode Island with Roger Williams and the other original settlers of Rhode Island; that Ralph and Joyce likely had children in Plymouth whose identities are unknown; that Thomas is the right age to be their child; that Ralph appears to have died when Thomas was a boy, making it plausible that he was apprenticed because the widowed Joyce could not care for him; & that there are no other known Wallen/Walling families in Plymouth or Massachussetts Bay who present likely alternative parents for him. If any of those premises turn out not to be true, then that might open up some avenues for further research on this?
