Wheewh!
Hard to watch the little one, while you have an even smaller one to keep an eye on.
I worried a lot about 'that my kid will know that I mean what I say' - but then I only had one, so that was a lot easier. You are 'allowed' and even encouraged, to act as obstinate as he is.
If you say "leave the Gameboy home" and it gets into the car, you are allowed to pull the car over until the Gameboy is in your hand...or turn the car around and go back home.
It's always easier to have a confrontation where you control the situation, than it is to get to the party and add in the public humiliation.
You are also allowed to put all electronics in 'lock down' if they are so enticing that your child isn't obeying you.
It took me a long time to get comfortable with the idea that 'I'm the parent, you are the child - You must obey me.' I kept expecting my son to be more like how I was as a child (a stern look would have me feeling terrible.) It isn't uncommon for boys to be very different indeed.
There is great appeal to explaining why, but if you are expecting your son to be able to go as you say because you gave such a good explanation, then you are expecting more from him then he can give at this stage of life.
I think you'll get more milage from pulling over the car - insisting that he put the device into your hand, and then asking him why he thinks it's a bad idea to bring a Gameboy to a party.
Disobedient behavior is very distracting to the driver.
I'm assuming in all this that he wanted to go to this party. If he doesn't actually look forward to parties and want to go in the first place, then that's a more difficult situation. If he isn't really that interested in the first place, then you'll have an idea of why you are bending over backwards. Some kids just hate birthday parties, that is ok. My son's best friend hates birthday cake, and his mom always provided a corn muffin for him to eat at his birthdays. I have a great picture of this kid at age 8, in full party hat and ballons, looking dejectedly at the 'first slice' that was served to him at one of those party gyms, before his mom made it over with the corn muffin. Parenting is so different from what I expected!
Love and More Love,
Grinity