David is quite passive and ready to runaway, he barely speaks and avoid speaking about why he moved, he is quite different from the other Evens. In fact he tends to take Isak's gestures/action instead of Even's one. It doesn't make him isak. It doesn't make him a sub. Lemme explains..
The way he acts remind me the way young trans people – who are still uncomfortable speaking about their transidentity – tend to avoid making contact with people they may end up liking/crushing on. Because they don't feel comfortable with the way their body is translated by gender norms they had to deal with their whole life, and afraid of their (potential) partner's reaction.
Which makes me sincerely hope they didn't only choose a trans actor but made the character canonically transgender.