hi there. my son also has a pragmatic language delay. i agree, it is quite upsetting to see your child frustrated when trying to communicate. my son is now 7 and has come a LONG way. the magical age for him was 4. At 4 he just started to really become much more verbal. i don't know if your son is receiving any therapy but you should certainly seek out a speech therapist.
my sons therapist is quite demanding of him and after a few sessions he knew what to expect. since pragmatics are conversational skills, he knew that when she said something, he would have to say something back. we also worked with him at home. i always had luck with subjects of interest and silly type of stuff. (ie. does a cow say meow?) he would find that very funny and correct me.....before i knew it, we would be having a (simple) back and forth exchange about that.
please do not settle on a speech therapist that just goes thru the motions. we have had many therapists come and go and one thing i learned a long time ago was that my son's time is precious.
my younger daughter is also such an amazing sibling to her older brother. since she is just naturally motherly (ok. maybe some would say bossy
), she helps him quite a bit and is always happy to play with him.