Your best bet is to call an attorney for advice.
It sounds like he is a "tenant at will". There is no lease so basically he is there on a month t month basis.
The best bet is to give him notice to leave in writing. Give a reasonable time frame. IF he does not go you may have to evict. This would involve the owners of the property. Which would be your parents. I would advise AGAINST doing things to make life difficult for him as there can be severe consequences. So do not have cable turned off or electricity etc. (Sometimes people resort to this when they want to avoid confrontation) Again, do not do that.
It is worth spending 15 minutes with an attorney so that you fully understand your rights and more importantly your parents rights.
Dealing with friends and family is very difficult. Best of luck to you in this.