Turns out, "belted earl" or "belted" is standard feudal terminology. Examples: "a belted earl"; "a belted knight".
Definition: An earl whose title goes back to the period prior to the 18th century when a sword and belt received from the monarch were tokens of that title.
Or: A man who has been knighted and received a belt and sword as the tokens of his knighthood.
The term was also used for knights, and probably could apply to any noble who took up arms.'
https://en.wiktionary.org/wiki/belted_earl
https://en.wiktionary.org/wiki/belted_knight#English
"By Edward III’s day, a knight needed to have income of at least £40, and knights needed to actively claim their status – hence the term ‘belted knight’, a man who had claimed their rights."
https://thehistoryofengland.co.uk/resource/glossary-of-medieval-terms/
>Why was he called "The Belted" and why was his son called "The Belt"?
I should have said "le ceinture" not just ceinture, means the belted one or something to that effect.
Because the French language does not follow the same grammar rules as English, and relies on context at times, such as in this case.