I am so concerned & unhappy.
DS is 6 & his behaviour is becoming increasingly bad to the point where the grandparents are commenting on how defiant he is 
We've had a number of days out this summer hols & it has been hard work from the minute he wakes up. Asking him to get dressed or put his shoes on ends in him losing his rag. I ask him nicely, I then tell him we need to hurry because we have to go, meanwhile he is telling me I should do it for him! I then give him to the count of 3 or he will be on the step for 5mins. I get to 3 tell him to go on the step & then he's begging & pleading for forgiveness. When I say no, I've given plenty of warnings he then becomes aggressive & quite honestly nasty 
I end up manhandling him to the step & closing the lounge door, he will then start throwing shoes at the door!!
Eventually he calms down & I talk to him about how none of that needed to happen & I say lets forget about it & have a nice day. But by the time we've arrived at our destination he's already managed to upset his younger brother (age 4) on the journey & I'm filled with dread.
The day then seems to be dictated by his behaviour with regular tantrums & aggressive behaviour, he will storm off if we say 'no' to anything, got himself lost once by doing this but when we found him he wasn't upset just embarassed & cross.
When we get home & its bath, stories & bed he will then refuse to stay in his room, I say 'get into bed please' & its a smirk & a 'no' from him, this goes on until I threaten him with losing his stories for the next night, again he'll push this until he loses them & then we repeat the begging for forgiveness & then the aggression again - It is draining & hardwork .
He has no fear, doesn't care if he causes a scene (yes, I do) & doesn't seem to care about anyone else.
One-to-one he is a different little boy, caring, considerate, helpful & it breaks my heart that no one else sees this. He is a clever boy, in the top sets in his class although he has struggled with friendships at school & is unbelievably competitive, tried to explain to him that there is no I in Team 
I would love to know that we're not the only ones going through this or if someone has any advice on how to manage his behaviour we would be so grateful.
Thanks in advance.