My ex girlfriend informed me that there's a 50%chance her son is mine... but refuses me to do a paternity test because she doesn't want her now husband to find out she has 2 possible fathers for her son... what can I do to enforce the paternity test?

How long has it been since the child was born and what state is this happening in ?
In some cases you may not be entitled to challenge paternity and the presumed father of the child is the husband. In some states they allow for a challenge from an outsider. In other states it can not be challenged unless it is by the mother or legal father (husband).

It depends on your location. The state laws vary from state to state.

Bottom line is you will need to go to court to establish paternity. Whether you can depends on the answers to the questions that were asked.