I'm sorry to hear about your sons problems - have you seen this information about NHS orthodontic treatment?
http://www.nhs.uk/con...ges/Introduction.aspx
As each decision whether the work can be funded on the NHS or not is done on a case by case basis I would recommend getting a second opinion because even though your current dentist has seen the problem as a grade 2 another dentist might see it as grade 3 (and therefore fund on the NHS). The information on the link talks about how much the teeth protrude at the front - if your sons problems are within the mouth maybe your dentist has failed to consider this and a full explanation (and your fears of infection etc) might persuade him to regrade?
good luck and all the best
IggyB