Your puppy is not quite done teething yet, so he bites because it feels good on his gums. Just keep redirecting to a toy, or a ball on a rope, or a tug, and it will get better soon!
That being said, it sounds like you're having problems when you first get home because your puppy is excited and is maybe not getting enough exercise during the day. How long is he alone during the day?
Another thing you said was that you untie him, so he's being kept on a chain during the day? Keeping a dog tied out on a rope is very dangerous, so if you're doing that please consider making other arrangements, like building him a kennel or even keeping him indoors in a crate. It's much safer for the dog! At the very least use a chain instead of a rope. The advantage is that a chain loosens if the dog isn't struggling against it, where a rope will tighten and stay tightened and can very easily kill the dog if he gets tangled up in it! Of course building or buying a kennel is by far the best solution! That way your dog can move around freely during the day, and there's no risk of getting tangled up.
It's not the size of the dog in the fight, it's the size of the fight in the dog. Mark Twain