R.B.
Hi R. - I'm a pediatric Speech Language Pathologist. There is definitely variability between children and speech/language development. Just to clarify (pediatrician's make this mistake all the time) - it sounds like your pediatrician is talking about language concerns (the actual words) vs. speech concerns (HOW the words are produced and pronounced) - but maybe both, I'm not sure since you mentioned that people don't understand him. Not to worry - but I think EI is a great option to help give him the boost he needs. Try not to be paranoid - it sounds like he has a decent base of words - and a variety of types (people, objects, adjectives, possessives, action, etc.). Generally we'd like to see children have approximately 50 words by 18 months. Of COURSE there is variability with this - some children blast past this (as your first did??) and some have much less - more like 20 or so, which sounds like your youngest...
However, I would at least get the Early Intervention evaluation. If they are so short staffed that you don't get anywhere with them, you can also seek out a private speech and language evaluation if you are referred by your pediatrician, at a rehabilitation center / clinic SLP. Also, another option is to check into your local public elementary school for an evaluation and possible itinerant services. I've provided it all - and I recommend the clinic setting as the best option if your insurance covers it - but EI is a great first step if they can get you the services/eval in a decent amount of time. I think the biggest complaint I've heard regarding EI is that parents and their children had to wait too long. EI is only provided up until age 3, and then you have to seek out the other two options if necessary. So it's best not to waste too much time - but ....
try not to panic through the holidays - and you can wait a couple weeks for your appointment. A couple or few weeks won't hurt him - honestly! Waiting months or a year could though.
In the meantime, just enjoy him for who he is - and don't worry about there being a problem. Sounds like a typical case from what you describe - and you felt yourself that he doesn't talk enough. BUT you also mention that he understands what you say, and has a great foundation of words.
Try to relax and enjoy the holidays!! Let me know if you have any questions. : )