If my fiancee died recently in a car accident and had money in his checking account, if I have his debit card and pin number can I get money out of the account?

No.  Once a peson dies no one has the power over his assets except for the executor in the case of a will or the administrator of the estate when there is no will.  His estate needs to be probated which will allow the executor or administrator to be officially and legally named so they can act for the estate and deal with his assets.  You do not have the power to withdraw that money any more than I do.  

In any event, as you are not related to him in any way, you have no legal right to any of these or other assests, but his son probably does. So you need to meet with an estates attorney to see if you can be named as administrator if there was no will and to protect your son's interest in the estate.