soul_words
I would suggest leaving the option for harem or no harem until later in the story when he's older. For now, I think him having flings and becoming more emotionally mature would be the best option as he's only 17 years old mentally. When he's older, the subject of harem could come up in story if he has multiple women he cares for and if they are ok with sharing him, but whether a harem would eventually come about or whether he would enter a monogamous relationship could be a good story beat for when he's more settled into life in the pokemon world and more established.