Hi Brooke-
The key here is consistency. Once you institute a consequence, continue until he folds and complies. A well-ingrained habit of tantruming to get what he wants or to express frustration will take a minimum of two weeks to break. You will not be happy, either, because you are the person delivering the consequence. You need to deliver it every time, without fail. Continue until the behavior is gone. Right now, he is clearly winning the battle of the wills. You are the parent; he is the child. He will not have a good life if the behavior continues. If he tantrums in the supermarket or a restaurant, take him outside until he stops. Tell him what you are doing, and why you are doing it. Do not allow him to hit you, if that is also a problem. You are on the right track with the sign language. Encourage him to use it. In general, it is best to deliver a consequence, rather than ifnore. Out of the house, removal from whatever it is until he calms himself. Inside the house, rather than putting him in his bed, put him in a 'naughty chair'. You will need to make sure that he remains in it until he has complied. You are bigger; you can do it! Use the sign language at the same time as you speak, and tell him how cool he is when he communicates with you that way. Never, ever, ever give him what he wants because you are tired or wish to continue with what you are doing. That is a reward.
All that said, make sure that he is always well-fed and has good nutrition. Very young children frequently need much more food than we think. Individuals may vary. Boys tend to need more food than girls, at more frequent intervals. Lots of poor behavior and tantrumming can be due to a hungry tummy or tiredness. Put him (and yourselves) on a schedule- same get-up time daily, breakfast, a.m. activities(encourage him to 'help' you and praise him. Same time daily for lunch, nap, dinner, bath, story, and bed. Stick to it. Children are much happier with a routine.
If, in spite of your absolutely consistent behavior, scheduling, etc. he is still doing the behavior after a month or so- at the same number of times daily (or more; you need to track this, and don't equate little snits with big tantrums - as you know, they are not the same), take to a pediatrician for a complete work-up.
Hope it helps- S.