To avoid it:
There will be a moment when you will have to call him. This will start a chain of events that can end in 3 different routes: Arato's, Satoshi's and Kendo's (the main route). To avoid going to Arato's route, you will need to NOT ask Kendo to leave the resort when you are asked. Then Satoshi and Mari will go to the beach with Shizuka and Arato to have some drinks. You will have to drink the whole glass. Finally, after some night events, try to stop Arato from fucking Mary when the opportunity arises.
To go to Arato's route: make Kendo leave the resort, drink the whole glass, do not stop Arato.
To go to Satoshi's route: do NOT make Kendo leave, do NOT drink the whole glass and stop Arato.