Take a breath... I was a preschool psychologist for many years (at the Newport News PEEP program) and assure you that your son will think he's having a play date with three adults who are there solely to learn about him.
Typically, the psychologist or social will meet with you to do a Social History. If you don't recall specific milestone dates and birth information, bring the baby book or make some notes! They will want to know everything- including any family history or learning or language delays. Ask your mother and MIL before you go!
There will also be a speech pathologist and a learning specialist present. They will engage Roman in a series of activities to measure his overall cognitive and language development. They will be taking notes and asking him to do things that are going to be difficult at times. He may get frustrated, but they need to see that too!
You will not get "results" today. They need to look at his skills as compared to his same-age peers nationally. You will be invited back to go over the reports and results. They may have some general feedback today, but nothing specific.
Remember that Early Intervention is very powerful and has a high success rate. You are catching this very early. It is far easier to teach a 2 year old to communicate than a 4 year old!
Waterproof for sure. I can count on one hand the number of mommies who weren't teary-eyed during the process.
Good luck and please do not hesitate to PM me if you need more details or want help with interpreting the report when you get it.