Quoting Constant Exhaustion:" It was one of the reasons why i avoid that park that we went to. It's just easier to get to when you ... [snip!] ... but that wouldn't really be teaching him that NO=DANGER. PS: yeah, i think my son was around the 18-24month level as well. "
Unfortunately there's no way to "make" him understand when his mind is that young. If you keep telling him no and explaining why, it will sink in but how much time that takes, there's no way to tell. He might understand tomorrow, he might not a year from now. Autism and special needs are tricky. It's definitely a challenge.