Lea, to clarify, I think you should have the assessment. But I don't think you should worry. Even if he is delayed, speech pathology can really help if you start early.

In the meantime, you can help him to improve his vocab by encouraging him to use different words, and longer phrases. For example, if he says "milk" try to get him to say "milk please" and keep repeating that phrase. When he gets that you can lengthen it to "milk please mummy" and then "may I have some milk please mummy" etc. Even if he can't say the different words or longer phrases, keep repeating them yourself. He will get there.

GL hun. I hope the assessment goes well.