Hi J.,
First, just take a deep breath and know that this will not last. During this 2-3 year age, everyone I know has had periods where their sweet child disappears and a tiny terror is in his or her place. But it is usually just for a few days or a week, especially if you put a plan in place and act on it consistently. All kids go through periodic "clingy" stages and you can comfort them while taking steps to re-establish independence.
Second, being consistent and loving about how you tell them the behavior is not acceptable will shorten the time period.
What has personally worked well in our house is for me to first explain what the appropriate behavior would be, and ask her to make a choice. Then if the inappropriate behavior continues, I give my daughter "room time." Some people use a naughty step or a time out - I don't think it matters where or what the space is for them to think about what happened. The difference in how I do it is I don't specify how long (time wise) she needs to stay there. She just needs to stay in her room until she can calm down and think about making a better choice. When she is calm and ready to behave differently, she can come out. She still loves her room and plays in there on her own - it's just her space to get herself together again. The nice part about this approach is that the first couple times, my daughter was defiant and stayed in her room for 20 minutes (a lot longer than the traditional Time Out for a 2 year old, but it was her own choice), and after a few days, it only took a few SECONDS (a lot shorter than the traditional Time Out!) and she'd calm herself down, come out and apologize. Now I usually just need to say "Do you need some room time to think about this?" and she usually says no and adjusts her behavior.
Third, try this if he really wants your attention: "Okay, mommy is going to play with you for 10 minutes, then she needs to make a phone call. What would you like to play with?" Give him your full and undivided attention, really have fun. Then near the end of the 10 minutes, say "One more minute, then you can continue to play with this and mommy is going to make a phone call." If he then throws a tantrum, see above!
Finally, you've ruled out tiredness and hunger, have you ruled out an ear infection or other health-related issues?
Best of luck! Warmly,
L. Still
Life and Leadership Coach
www.buoyantlife.com