You have to find Stitches outside& have him ping when he sees you. Do you have any side characters other than your mayor? If you do, save and quit on your mayor and log on side character, and check if stitches will ping him or her. If you only have your mayor and you don't mind TTing a few hours, change the time and TT like an hour ahead or back. Log on and find him outside. You can probably keep doing save & quit and load up the game til you see Stitches outside w/o TTing... Or you can also try wetsuit/diving method. (Basically, fill up your entire pockets with flowers or seashells, and go swimming in the water for 5 minutes or longer and run in front of Stitches so he will ping).