I wish I could say I have, but I've been spending most of my money, mostly on video games and pizza.
This statement implies you should take trading a lot more seriously PRIOR to trading anyone's money.
Simply, stop the video games and pizza stuff so that you can grow your own account via moving from 1 to 2 contracts for a few months. If your account keeps growing...then go from 2 to 3 contracts for a few months and so on until you reach 5 contracts.
At 5 contracts and if things are still going well...tell your grandpa to open a trading account with your broker and then you LINK your account with his account so whatever trades you execute in your account will also simultaneously occur in his account.
The reason why I say to go slow is because MOST traders have a psychological problem when moving from 1 contract to multiple contracts especially directly from 1 contract to 10 contract...resulting in a negative impact on your trading. You can only find out if you're one of those traders or not one of those traders via moving slowly your trading account as described above (1 to 2, 2 to 3, 3 to 4 and then 4 to 5)...
You can only do such via growing your trading profits via a little compounding instead of spending your profits on pizza, video games and whatever.
Personally, I wouldn't let any one touch my money with your
inexperience level of trading
multiple contracts and after hearing what you've been doing with your profits so far instead using those profits to grow the trading account so that you're independent and taking life more seriously beyond pizza and video games.
I just think your grandpa sees what is going on with your life and he has some extra money laying around and it just offering to help you in hopes you'll take life more seriously via paying you a salary
as if you had a job.
Simply, your grandpa is trying to motivate you and you should be thankful you have a cool grandpa.