I have just gone down the Paediatric Physio route with my 3 year old. This was through the NHS and we had to wait about 6 weeks from referral. I know that it can seem a long time to you to have to wait, but I think it would be worth going down the NHS route, as they do seem to be getting more knowledge, and could you afford to pay for al the Physio etc that he may need, otherwise you would have to wait for this through the NHS anyway, and would probably have to go through your GP to get a referral after getting a diagnosis privately.
In our case my 3 year old isn't too bad at present, and we are doing the watch and wait thing. I did ask about my 8 year old while I was there though, and the Physio suggested that I should get him referred to him, who will assess and decide if he should be sent to Newcastle to see the Paediatric Rheumy and the specialist Physio and OT etc, to get a full care plan and then come back to the local team. It could be that they have a similar system in your area, where the Physio assess and them can refer to specialist teams. As they aren't even sure if it is HMS that is causing your sons pain then this would probably be the best route, as then you can be referred to the right people, without the expense of you paying out to see someone who will just say it's not whatever they specialise in. The Physio we saw was very knowledgeable in lots different conditions, and surprising knowledgeable in HMS.
I hope that you get some answers for your little boy soon, as I know how hard it is to see your child suffer and not know how to help. I take it that you are giving him Calpol etc? Also have you tried heat packs/pads? I find them very helpful and have an electric one that was pretty cheap from Argos. If he is struggling with school then maybe you could arrange a meeting with them and discuss him only doing part days, or having somewhere he can go and have a lie down if he gets too tired or in too much pain, with lots of cushions and heat packs. You can Flexi-school, which mean part school and part home educating, it can be very useful in cases where children are struggling at school due to pain/fatigue as it means they can have a break and do work later in the day, or over weekends. The coldness thing can be common in HMS, I have always complained of it being cold and everyone else complaining they are too hot as I have the heating on full, wrapped in jumpers and blankets. I am not sure why it happens but my personal theory is that we use so much of our energy just staying together that we can't keep warm too, or that it is down to useless circulation that many of us also suffer from. Hot baths and heat pack are useful for this as well.
Hugs to you and your son! You may not be suffering the pain like your son, but I think the pain of seeing your child suffer can be just as bad!
Bex! x x x
Finally diagnosed March 2011 after 20 years of hypochondria!!!