Thank you all so much for the replies, I feel much better about things now. Waiting for the private speech therapist to get back to me after she has spoken to DS's NHS therapist to see what kind of plan they've got for him. That will be interesting, because I don't know if they've even got a plan.
DS's problems seem to me to be mostly physical, ie. not being able to produce the speech sounds, but I feel there is something else going on as well. Sometimes he can make a sound perfectly the first time he tries, but he cannot reproduce that sound correctly. Most of the time he cannot tell the difference between two sounds, like 'g' and 'c' . He cannot blend sounds very well, if at all and gets easily confused/can't hear which sound he is supposed to be making. Also, he still says Me am, not I am.
DS works very hard at speech therapy, always co-operates and is willing to try at home and at clinic. He has made virtually zero progress, still working on the same two sounds he started on at age 4.
DD (9) who started therapy at the same time as DS has come on in leaps and bounds, but, she still has areas of concern so needs another assessment. For example, she says 'hellbellls' instead of elbows, 'afused' for confused, 'bisketti' for spaghetti, some letter sounds are still unclear but on the whole she is understood most of the time.
If DS could get his speech up to the same level as DD, we would be delighted, but, if he is unlikely to make progress, then I would like to stop therapy for a while, give him a break and take the pressure off him (he has so many other health problems and we always seem to be at hospital/clinic appointments) and maybe think about communication aids or some other ways to communicate.