Fast flow

Who else has a fast flow or let down and how do you and your little ones deal? I'm trying to identify when my let down occurs as I can't always feel it or at least I don't recognize it. But my son who is 3.5 weeks old has a hard time feeding sometimes and o think it's too fast a flow, not too slow. I try to lean back or help him be above my nipple but often times he's already overwhelmed and the feed takes a longtime and I fear he's finishing before he's full.