hiya, my dd2 was like this, her tantrums started around 18mths,
I had visited the hv quite a few times and she just said 'she will grow out of it' 'try using a reward chart' we tried the chart giving gold stars for good behavior and black spots over the gold stars for bad behavior and if at the end of the week she had 20 gold stars then she would have a comic, dd2 decided to colour it in black so she could be naughty. My dd2 also has no fear of danger, and if i go anywhere i tend to dress my 2 dd's the same so i know what she is wearing.
when my ds was born she was three and a half, and i couldn't take it any more. i went to see the gp and just started crying i couldn't stop i explained everything it had got to the point i would go through daily motions with her, feed, wash, dress, take to nursery, tried not to talk to her otherwise she would have a tantrum, i didnt like her and to admit that it makes me sound horrible, but she would scream (no tears) for anything from 1hour to 3 hours at a time, there was never anything that would calm her down, the gp, firstly tried to tell me it was because of the new baby, but it wasnt she was like it before he was born. They then referred her to a child psychologist, because she was like it in school aswell, we managed to get a cancellation appointment and was seen two weeks later, we were asked lots and lots of questions to which i answered, on the first visit they said that they could rule out adhd/autism, and to try for two weeks a new diet, taking out all processed food, sweets, pop, chocolate. I cooked all foods from scratch. and to be honest i found a difference within days. we went back to the psychologist and stated that we had seen a big change, they think it was all of the additives that were in processed food.
It has been 3 years and she is now 7 and we still have days where she is in a mood, especially after she has been to a friends and has had pop, or sweets. but life is alot better, calmer.
hope that helps, go and see your gp, or hv, and ask for advice you know your son, make sure they dont fob you off