How can I save madd dogg when he jump quickly

Stay still until he actually jumps, then move the truck.
If not, use the slow time cheat so you can be more accurate.

Just follow this tips:

Head over to the Royal Casino to find Madd Dogg a little out of his mind and
Threatening to jump. What you need to do first is quickly run over and grab
The pick up truck with all the boxes in it and park it under Madd Dogg. Now
For the next part it's kind of a ltitle minigame in which you need to
Accelerate and decelerate (turning is not necessary) to stay as close as
Possible to Madd Dogg's position. Eventually after he's stumbled around for
Awhile he will indeed jump. You don't need to be 100% accurate but you do
Need to be close. When done correctly he lands on the back of your truck with
About half his health, do the right thing and rush him to the hospital as fast
As possible. It's similar to the paramedic missions in that you need to avoid
Bumping too much so that he survives the trip. With no time limit simply take
It easy and drive carefully to ensure Madd Dogg gets to the hospital okay.

Hope this helps! Good luck!

it's easy just don't do this!!! Don't kill your GF Denise it's all!!! I kill Denise that happen he jump fast but if u don't he will w8 for the truck!!!

Why can't you guys help me save madd dogg you guys should please help me pass it thank you for your concern

You can use the website SaveNPlay to fix your savegame, I also had this glitch before it has something to do with saving the game with the riot cheat activated or 500 overall cheats activated.