My character is about to fight someone that really strong but he believes that he is still weak. So how can i force him to fight. (For plus note: i make a fantasy story like magic and stuff)
I think you might be focusing too much on a plot device and not enough on character development.
As Artichoke mentioned, your character doesn't need to fight. If your character is so weak that he would never fight, there's no way to force him to do so.
You could make him run and hide. Even though the enemy might pressure him to fight (such as endangeringl his family), if your character is really very weak, he might still hide and then live to regret his actions.
If you really want your character to fight, you have two choices:
Change his character. Don't make him so weak.
Give him an experience where he grows and becoemes capable of fighting. For instance, maybe he fled and regrets it for the rest of his life. Follow him for months (or years) after this incident, and make sure he has constant flashbacks. Then, a long time later, something similar happens... and now he's finally ready to fight.
Don't rush the story.
As @colmde has commented a character may be forced to do something.
But what I feel you mean is that the character is strongest than he thinks. So It could be a great chance to prove him wrong. As many things in life you think you have some limitations that are not real.
An accident or stressful situation could make you grow or erase this mental blocks. Or perhaps the threath is not as dangerous as It seems. Many times people overestimate a lot of situations in Life.
Kind regards and good luck with your writting.
Everyone who fights must have a reason to fight. For the bully, it is to maintain his/her dominance. Your character, on the other hand, would need a strong reason to fight when he believes he is too weak to do so.
Strong reasons can include survival of self or loved one or tribe, extreme anger or betrayal, or a lack of other options--being cornered as @colmde has said previously.