How to help grandparents

*You* can't do anything other than, if you become aware that he committed a crime (e.g. stole from them; attacked them), you can report that crime to the police.
Your grandparents have the right to make him leave: based on what your write, he is a guest (not a rent-paying tenant) in their home, and a guest can be asked to leave at any time. If he won't, they can bring an action for ejectment ("ejectment" is eviction for nontenants) to have the court remove him: they can get a court order than will cause a court officer, constable, and/or sheriff to remove him physically if he won't go, but they need to bring the lawsuit and get that court order to do this.
(Note: an action for ejectment is more procedurally complex than a regular eviction; they should retain an attorney to help them.)
If matters are urgent (e.g. threat of violence), they could even potentially file for the action on an "emergent" (think: "urgent" or "emergency") basis to get into court faster than usual to get the order removing him. (Emergent actions increase the complexity; again, they should get an attorney.)
They could also possibly seek a restraining or protective order against him if he has demonstrated that he is a threat.
But they--your grandparents--have to do this, unless and only if they were to give you a power of attorney giving you authority over their home; then you'd have the legal power to ask him to leave for them. Otherwise, they are mentally competent adults; they have to act on their own behalf. 
If they are willing to give you a POA, have an attorney draw it up and make sure it is signed and witnessed property, so it is effective.