First of all, check Dunsparce's friendship! Call Bianca with the Xtranciever: if she says "What great friends you are! It's so nice how you trust each other! It makes me kinda jealous!" he's at 255 happiness, otherwise no (to see how far you are from 255 you can refer to this list
of Bianca's statements.
I never used the Beauty Salon in Join's Avenue so I don't know how it works, but usually the services/item sold in the Avenue are avaible once a day.
You can still raise his happines by giving him a Soothe Bell and using vitamins (if not EV capped and if the selected EV is below 100) and/or EV reducing berries (which are sold by a women over the west exit of Nimbasa City). Walking with Dunsparce in party and leveling up are the longest way to raise his happiness, but they still work.