Please help desperate mom of four..2 year old woes!?

Sounds a lot like 'Middle Child Syndrome', and I don't envy you. Most techniques to deal with it involve a lot of one-to-one time with your son, and I'm sure time is something that is in short supply with you at the moment! Praise him a lot for anything good he does, and if possible, designate a regular weekly time (even an hour) to do some activity with him alone, just to reinforce in his mind that he is special and loved.