What to do about self-esteem issues in younger kids?

Really looking for some guidance here: I have two sons, 6 and (almost) 4. The 6yo is the one I'm worried about. He is very bright, but is constantly negative. He will constantly put his brother or himself down, and he'll start it with whispers, so it seems extra malicious. We only know it's happening because the younger one will yell out "I'm NOT stinky! I'm NOT yucky! I'm NOT stupid!" We've tried asking him nicely to not be mean to his brother, and he immediately redirects it to himself: "Fine! I'm the yucky/stinky/stupid one! No one loves me! Throw me in the garbage!"He always looks to the negative, which is hard because my wife and I aren't like that at all, and neither is his younger brother. He's constantly saying he can't do things and that he's not smart, which is really heartbreaking because of how smart he is. He could read at 2, and we didn't really teach him. He just could. It was amazing. But now, he insists he "doesn't know how to read" and will only gravitate towards very simple baby books at bed time.My wife and I are already looking at family therapists but in the mean time, if you have any experience with matters like this I'd love to hear about it.Thanks! via /r/Parenting https://ift.tt/2UTocp9