I triggered it by visiting him at the VA school and there was a lost dog. At the end of that scene, the bear from the love hotel offers you the dog costume, which you can visit the hotel at night to buy. After a few days or so, the even will trigger.
uhmmm i got it one day at a cut scene some events only trigger once in a blue moon, so keep spending time with him and the bear will show up randomly and offer the dog costume for 20k then spend a day with Shiba again and scene will/should be triggered
