I've had this problem previously and I wasn't able to fix it. I didn't even know why he wouldn't join me. I've figured out why he won't join me but I don't know how to fix it. Usually to recruit him you ask about the GECK and eventually he gets to talking about wishing he joined the Vault Dweller, at which point you can say: "I am following in my ancestor’s footsteps. Would you like another chance? Will you join me?" That option however won't show up for me. Instead I get: "I’m sure that the work you do here amongst the ghouls has value and meaning. It was good to hear of my ancestor. Let me ask you something." I thought maybe this was because of the restoration project but that's not it as these quotes were copy pasted from a transcript of the base game. Said transcript doesn't tell you how to unlock each option. I thought maybe it's because I said something wrong in Vault City or Gecko so I loaded a save where I hadn't talked to anyone in either location but that didn't work. I'm not at my party limit as I installed a mod which removed that and even if Lenny was unaffected I had only 4 followers. Although admittedly I hadn't tried getting rid of my followers just in case but I heard someone else on a different forum tried but it didn't work. My character has an intelligence of 8, so I don't think that is a problem. If I recall correctly you don't need to do a quest to get him with you. The only title I have is Virgin of the Wastes, so it's not like he dislikes me. I have no clue what needs to trigger to change that line of dialogue, does anyone here know?