TL;DR: I have a player who wants to recruit lots of NPCs to avoid putting his own character in danger and to grab more spotlight for himself. I'm concerned about how this will affect challenge balance and the amount of fun the other players have. He doesn't want me to run the NPCs either.
I recently started running Lost Mine of Phandelver as a new DM with a group new to tabletop RPGs. One of the players is a long time friend of mine but he decided he wanted to try to recruit a goblin into the party temporarily. It was the last one of a group that they killed and I responded that they could try to persuade the goblin into fighting for them. He rolled real well and now the goblin follows them. Naturally it will most likely die or something else will happen that will make it no longer part of the party but, when I was talking to the player after the game, he went on that he would just try to recruit another creature or NPC they encounter and if he failed he would just keep trying with other creatures/NPCs.
Now for background I know that he enjoys playing games with a sort of overpowered sense. He likes to play god a little as well as have others get their hands dirty for him. (Example: In Skyrim he uses mods to have tons of followers that fight for him) He doesn't care for a challenge that has real consequence. He prefers simulated challenge (he also uses mods to keep followers from dying so he can still have a challenge of a difficult encounter but it will never end in complete failure.) He has told me that he prefers this style of play and it doesn't take away at all from the "fun" or enjoyment of the story and gameplay. He likes to take the spotlight and appear as a cool or respected or even idolized by others that may look up to his "greatness".
So my question is how should I handle him wanting to constantly recruit non PCs into the party. I don't mind the occasional NPC tagging along if the NPC's goals can be met by doing so... but I don't really like the idea of constantly having him persuading non PCs into the party to fight for him, especially if he does it to give him a sense of control over the party and take the spotlight as a leader all the time. I'm also worried a little bit about balancing and if the other two players will get annoyed with this. They seemed fine with this one instance but what if later down the line they grow tired of the concept and it steals from them trying to enjoy the game.
When I confronted him one on one about it and he told me his future plans I tried to gently let him know that I really wouldn't be comfortable with this being a regular thing and stating that as a new DM I'm not 100% sure how to balance and make sure the other players don't feel left out or over shadowed by his play style. Let's just say he grew rather upset of me ruining his fun.
We never came to a conclusion and I'm not sure if I should just let him do what he wants and just keep losing and gaining followers in the party or should I be firm about this and say it can't continue to happen more than the occasional NPC doing it for the NPCs own reasons or if I should create a permanent NPC follower. One of the problems is it can't even be a NPC I control as a DM, he wants to control it.
I'm new to DMing and this is the first problem I've run into where I'm not really sure how to rule on it (and I've been very liberal with the "Yes you can try to do that" attitude) I would really appreciate any advice.