How to fix a slacker boob?

My baby never seems full after being on my left boob, he will stay on it for 20+ minutes and still cry for more and get frustrated towards the end, pulling off a bunch. But he usually milk-comas out after switching him onto the right.

I don’t have an electric pump at the moment, just a manual and I don’t use it much yet.

What’s the best way to help amp up my production on the left? Or should I just continue to feed on both sides every time?

Pic of my little man for fun 😊