Clean and nice decor shows me the owner is serious about maintaining their property and looks after it. It doesn't need to be modern but it needs to be fresh and suit the property even if that's a vintage style.
Warm hosts is a nice thing. And a good breakfast is my favourite part of staying away.
Comfy beds with spare pillows and blankets in the room for you to use if you need them.
Things I've appreciated staying with ds before- one place provided a little pop up play pen in the room (lone parent so this was helpful for showering). One place left a little soft towel and bath/shower goodies for children in a little gift basket which ds was delighted by. Child friendly options on the menu that aren't just chips and [insert frozen food here]. I get why these are crowd pleasers but ds is a good eater and the last time we went away I found it really difficult to find a kids menu that included a vegetable.
Having a kettle and fridge in the room is always really useful.
Information in the rooms of things to do nearby and some bnbs near me have agreements with some local attractions and will give a do a discount on tickets.
Somewhere to play outside, secure and with some kids toys/ swing/ slide.
If he's hosting families then it means he needs to be thinking about how soundproofed the rooms can be. Last time we stayed away the guests in the room next to us were up chatting and laughing until 3am. They weren't excessively loud but the walls were obviously thin which made it hard for me and ds to sleep. Similarly I wouldn't want to be next door to a crying baby.
Is it worth him doing local surveying of tourists - do they want dog friendly, no dogs, family friendly, child free etc to see what the market actually is.
Does he do any entertainment or specialist hosting?