Barking madness!

I love my dog. I got him last year in May, and he's such a sweetheart.

I take him on at least 2 walks daily. (generally lasting at least 30 minutes or more.)

I've been to a public dog training class and he's so well behaved in public!

Except...

He barks. At everyone and everything. He barks at the wind, he barks at the kids coming home from school! He barks at my cats!

(He's really well behaved with the cats most of the time doesn't chase them or anything. Sometimes in the middle of the night I think they startle him and he kinda like slams his paws and makes a Harumph! Sound)

He barks at my uncle when he comes to visit! He barks at my sister when she comes around!

He's not aggressive towards them if he's barking and they approach. In fact he will hide behind or beside me if they do approach.

The only person who has successfully got him to stop barking for a time period was my boyfriend. Who was visiting from his home for a little under a month, and he did that by everyday having my dog follow the commands I've taught him. Sit, touch, go to grandma, Look, down, one of those then treat. He was on Vacation and had the time to dedicate to that.

But my sister and Uncle aren't here often enough to do that and his barking is actually becoming a deterrent for people to come into the house.

I've tried teaching him speak. However, he won't for some reason bark on command. I thought if I taught him speak, I could then teach him Quiet.

What can I do to help him understand that not everyone who comes into the house needs the thousand bark greeting.