This speech stages among children is helpful: https://sg.theasianparent.com/when-do-babies-talk I can say your kid is on track if he can already follow simple instructions.. my daughter was also still mumbling when she is that age. I saw the huge bump up on her vocab when we started to speak to her constantly, no baby talks and also by reading story books to her.
Continue conversing with him as how you would with an older child. I avoid baby talk so as to not encourage him to mumble even more. I also tend to exaggerate and enunciate some words in hoping that he will slowly follow.
Baby loves facial expressions so it helps when you exaggerate them a little. And engaging in baby talk with a slightly higher pitch could help too.
maaf saya tidak bisa berbahasa inggris
Talk to your baby often