I talked to the son at one of the early (~2008) YC dinners. Actually found him more approachable than PG or most YC founders; RTM is a nerd in the "cares a whole lot about esoteric mathematics" way, which I found a refreshing change from the "take over the world" vibe that I got from a lot of the rest of YC.

Interesting random factoid: RTM's research in the early 2000s was on Chord [1], one of the earliest distributed hash tables. Chord inspired Kademlia [2], which later went on to power Limewire, Ethereum, and IPFS. So his research at MIT actually has had a bigger impact in terms of collected market cap than most YC startups have.

[1] https://en.wikipedia.org/wiki/Chord_(peer-to-peer)

[2] https://en.wikipedia.org/wiki/Kademlia