My ds has a speech disorder.
When we went to the speech therapist she recommended talking less to ds.
When someone doesn't talk adults naturally start to ask them lots of closed questions and start talking more and more themselves. This can then reduce further the amount the person talks. Similarly putting pressure someone to talk does not help.
My ds also has a processing disorder so whilst he did understand everything said to him he then struggled to recreate the individual words himself so needed to hear individual words in isolation.
She recommended spending some time each day playing with ds at his level and initially not talking. Waiting for him to say the first word or sound. Then to repeat that word and add a single word to it (eg if he said car I would say 'fast car' or 'red car' or 'car down' etc as time went by to add more words as he added more words
We did this every day for a short period each day and it really helped.
However my ds also has dyspraxia so struggled to make each sound. She said that research showed that the most effective time to work on speech sounds is when children are 6. We then taught him the speech sounds, taught him how to combine speech sounds, how to put speech sounds into words, then sentences and then finally into spontaneous speech. It was a lot of work but we got there
Ds was assessed at about 9 and his comprehension was above average, his speech was within normal range and his grammar was below average.
There are some good NHS speech and language therapy websites with resources and activities.