If you have a very weak manager. Your colleagues are making use of the situation and manipulating him, what will you do?

I will work with him on his weak point(s) in order to give him a chance to prove himself .

Go one on one discussion with your manager.

Tell him his strong points and then politely mention his weak points

If you have any plans by which he can improve on his weak points, share with him.

Explain your colleagues that they should not take the weakness of their manager for granted, rather help him in overcoming it everyone should work as a team.

Finally if you see there is will issue in the manager on working on his weak areas and the team and the business as a whole is suffering then report it to the top management so an action can be taken.