I feel your pain! My oldest just wants to please everyone, so has always been easy to "discipline." (His issue is hypersensitivity and getting hurt feelings too easily - we're still working on this). When my daughter came along with the opposite personality, I was at a complete loss. She is now 2 and a half and suprisingly easier to deal with (despite the usual "terrible twos" stuff) than when she was 18 months. When she was your son's age, she threw the most unbelievable tantrums - also physical, biting, hitting, and just rage that she couldn't seem to get control of. I read numerous books and looked for advice from the internet, friends, relatives. I was actually becoming "scared" of my daughter - the strength of her rage - and anticipating the years of therapy we'd all have to endure and wondering how I'd ever deal with her as a teen. My mother (who has been an elementary school teacher for 45 years) kept reminding me: she will outgrow this! My mother suspected that much of my daughter's rage was based on her inability to communicate. So when she was 18 months(+), I just tried to stay calm during these rages, put her in a relatively safe spot, and leave the room until she calmed herself down (which could take forever it seemed). As she got a little older and started to say some words (and understand more than she could say), I started reiterating what the preschool teachers always did (I work full time too and my kids go to preschool) - telling the kids to "use your words" to get what they want (instead of hitting, crying). I would just keep pushing the "use your words" routine whenever she got uspet and try to help her find the word she needed (e.g. "Milk?" or need "Help?" or a "hug?") - and slowly, but surely, my daughter has learned more and more to use her words. She is still learning and needs constant reminders, but is sooooooooo much better than she was. The tantrums tend to only come now when she is over-tired. I think the root of it is just teaching kids how to express what they need in a healthy, productive way - and to try and maintain your sanity and patience until they finally figure it out! Also, I know the preschools have a set standard for disciplining the kids (the whole "use your words" must come from state guidelines or something, because all the schools seem to use it). Since your mom is caring for the kids during the day, it's probably extra important to make sure you and your mom (and husband, etc.) are responding to your 18-month old the same way.
Good luck!