I'm thinking of faking a relationship out of desperation and now don't know how to tell him
Why do you want to fake a relationship and not tell him about it?
I need financial support but I am not really into him.
You may have reasons that makes sense to you in faking a relationship with others but it is wrong to fake a relationship.  You need to be honest with the other person.
What if I say that it does not matter because he doesn't mind.
I doubt he would be ok with being used for money. He needs to know because he could get really angry if he finds out what you have done.