I agree with Angela that it will be OK.

At 18 months, I am not sure I would be terribly concerned about clumsiness and falls, but if they continue even after you work on hand eye coordination and balance then he should be checked for Neuromuscular Disorders (probably included in some of the genetic testing done). A diagnosis in this area isn't the end of the world, it just lets you know what things you can do to help your son at a young age live as "normal" a life as possible. I have a rare genetic disorder that runs in my family. I was "normal" until college. Poor diet, too much stress, and genetic predisposition all connected to make my condition annoying starting in college. I was ALWAYS clumsy as a child even though I was "normal". I had all sorts of weird quirks, but so did everyone in my family so they didn't stand out as abnormal until college.

It doesn't sound like your son has Autism from your description.

Taking your son to a playground should be fine. Him getting grubby won't hurt him no matter what health condition he might have.

If your child ends up with a positive test result for a Neuromuscular Disorder or any rare genetic disability, contact the Muscular Dystrophy Association. They will help make sure your child gets proper care even if you don't have insurance. There are similar groups that exist for some of the more common conditions like Cystic Fibrosis or Cerebral Palsy. A diagnosis will help you get the help you need. The bigger challenge is if your son doesn't have a clear cut result leading to years of doctors going "I don't know", that is when a condition gets expensive and difficult to treat.

As far as your son not playing with the exact things all other children supposedly play with at his age, play with him with other things. Some things are just not interesting to a child. Even when they are "normal". Everyone is different, and a guide is just a suggestion not the bible.