This is from the Fallout Bible so far. It was written as an ending narrative but wasn't added to the game.
"You still hear mention of Harold from time to time. Apparently, the tree growing from his head has gotten larger, and if rumors are to be believed, fruit is growing from it. The seeds are said to be remarkably tough, and several of them have taken root even in the most barren stretches of the wasteland."
I'll keep looking to see if there is any more info on our good old friendly mutant
*EDIT* That is all I could find that was mentioned other than Harold's back story within the time line.
I told the Overseer of Vault 13 I'd give everything to save everyone, and so I did, I saved them all. Since then I've felt loneliness.