Hi C.-
You have gotten some solid advice. At 18 months, he should use anywhere from 5-20 words. If he's not, the screaming could be the only way he knows to communicate with you. That being said, I don't think ignoring it or punishing him (ie putting him in his room) will help. He is old enough to use a few basic signs. I am an early intervention speech therapist and we typically start with "please", "thank you", "more", "yes", etc. Kristen's advice regarding choices was right on. In early intervention, we encourage parents to offer their children a choice between 2 objects. That way they can point to what they want. You can also take his hand and ask him to show you what he wants. When he does, you can say something like "Oh, you want milk." He will learn that this will get his needs met, and the screaming will probably decrease. Once he does start using signs, pointing, etc. THEN you can start taking a stand against the screaming. Until then, assume this is his only means of communication. What you don't want is for the behavior to get worse.
As for Childfind...If you are interested in having him evaluated for early intervention, you need to contact your local school district. They will give you the number for Childfind. Childfind does the evaluation. If he qualifies and you are interested in services, they are provided at no cost to you. If you have any questions, please don't hesitate to ask.