Hi Julie & Welcome.
Time & patience is whats needed here,from what you say i think charlie was probably bonded to the male & obviously not the female.It will take time for Charlie to trust you & gain his confidence back.As the others have said,sit near him talk to him ,about anything,the weather etc..
Is there any time in the day your around for a good few hours ? if you are then leave his cage door open,let him come out on his own accord,keeping him caged wont help the matter,as this point keep all his food inside the cage,when he is hungry he will return,but it may be a good few hours
Have a good look around the different rooms & use the search box at the top of the page,if you enter different keywords previous posts on the particular topic your looking for will come up .