I have collected 3 pieces of comet bloom for the side mission, but the old man is nowhere to be found. How much comet bloom do I need to collect? Will he come back?

Remind me again, what town is this in? If it's the old man in subway town, he goes into shop where you buy your supplies from, forget the name.

ok I found him in a doorway chatting with some other guy, but what was the reward? it said he would give me "something you could use" but I see nothing new in my inventory or the engineering section

I think all you get is money, i already had comet bloom when i first talked to him and right away it said completed and he still is in the same place. i dont know if i have to keep giving him more comet bloom

You get the ability to craft some health related thing.. can't remember what though.

Commet Bloom can be found behind the glowing feltrie near the authority's blockblade in the Eastern Wasteland..and the NPC for this side quest is Nobu, the old man standing right in the stair that lead to Redstone's office. And you ony need 1 commet bloom to finish this quest.