As people always say: All children are different! Writers and publishers make money from books that tell parents the various stages of development etc, but in practice only YOU really know your own child.
I don't doubt it is ALL sinking into his brain, and when he has sorted it all out - what is relevant; what he likes and doesn't like; who he can trust, and who he is wary of - then the language will come, and you may find yourself wishing he would be quiet for five minutes!
Some people advocate 'baby signing' as a way to early communication, and signing is easier than manipulating mouth and vocal chords at an early age.
What form does his access to numbers and letters take? Do you mean he can sequence number or letter cards in the correct order? Does he know what value the numbers represent, so could he place the appropriate number of counters, or Smarties, onto a number card? I rather doubt it.
Read to him, talk to him ALL the time, count Duplo bricks and tell him their colours. Avoid too much TV or other 'screen' activities; play in the garden, go to the park etc.
If you are still concerned after trying all these ideas, see your HV, but I doubt you will need to.