If you have something with letters / numbers on e.g. a book, you could ask him to point to them, like “where’s B?” or “where’s 6?”‘ etc. He might be able to recognise them even if he’s not able to say them yet.
It is not unusual for simple counting (1-3) to start even before words, so if you ask a 18M old how many blocks they have in front of them they would use their finger to touch each block as if they are counting silently. Also asking "where is" questions like the previous poster suggested is also a really good idea.