How to legally motivate an 18 year old unemployed uneducated son who refuses to leave the family home located in Montgomery County Maryland?

Other than flat out kicking them out, not much.

My suggestion would be to 'suspend' them from the house for awhile. Have them live on their own and see what it's going to be like when the time comes to finally part from his parents.

The son being uneducated makes it harder to accomplish that though since they are more than likely not going to find a job that pays well, or a job period for that matter.

If all goes well he should be running home to mommy/daddy begging to do better and to let him live there again within no more than a week or two.

I've seen this method work before with a friend's son who was 18 and quit high school, attempting to live off of his parents. It straightened him out, hopefully it will work for you as well.

I wish I could offer a better answer, but forcing humans to realize things is often very hard and frustrating. Don't give up!