There's litotes - a figure of speech consisting of an understatement in which an affirmative is expressed by negating its opposite. But that would be exemplified by "Bill Gates isn't doing too bad".
OP's "doing well for himself" is just plain understatement. If OP really wants a literary term, there's meiosis, but it's a very rare literary term these days (I have a degree in literature, but until I read Robusto's answer here I only knew this word in its biological cell division sense).
For anyone who wants to dig deeper, OED says it's from Gr. µείωσις lessening, f. µειοῦν, to lessen, f. µείων less. It's used in the "cell division" sense because the two "daughter" cells each have half as many chromosomes as the "parent".
Personally I'd avoid meiosis for rhetorical understatement. It's English, Jim, but not as we know it.