In the UK where you live, protocol is not to treat children unless there is good reason but if your son has overt bleeding symptoms then you should seek medical advice. Bleeding means blood in the urine or faeces, nosebleeds or blood blisters in the mouth. Petechiae and bruises on their own are normal with low counts. But if you are worried at all then contact the hospital. You should have been given a number to call to speak to a haematologist rather than just calling A&E who will not have the experience to properly advise.
Where are you in the UK? There are several centres of excellence for ITP in children.. the ones so far are at Manchester Children's, Edinburgh Children's, Birmingham Children's, Wales Children's, Oxford Uni, UCH in London, St George's in London, Addenbrookes, Jimmys in Leeds, and the Norfolk & Norwich.
You can also get support from the UK ITP Support Association..
www.itpsupport.org.uk/
I don't think the website is all that good but I've been to several of the conferences and they are great.
