I pick every quest that Preston has for me.
I walk him to destination and I watch him complete his own quests.
If he goes down, I get him up.
To celebrate him getting the castle I put up on the wall glowing letters, forming "Preston, we love you".
I had bunch of differently dressed mannequins put around castle and settlements to make him think that we have more people that we actually do.
I ended up cosplaying Silver Shroud just because at one point he said he likes this character.
Preston be praised.