Hes probably afraid you'll hurt yourself. My advice would be to lay an example down that you're going to be responsible with it. Though we all know you arent. Dirtbikes + sensibility dont often go hand in hand. lol
Show your father you're capable of being responsible. Not just washing dishes this time, but that you want to put your safety before showing off tricks. If you've got a bicycle or skateboard, wear your safety equipment (you know, helmet, knee and elbow pads). And ride responsibly. It might put him at ease.
Ask him to get you all the safety gear for your dirtbike too. And wear it. You'll likely regret it if you dont, bro.
If all else fails, buy one yourself when you're old enough to. Good luck kiddo.