hes faking it!

hi all, ive posted a few times on my own experience of dr's and my PRA. now im struggling with my son and im heartbroken.
a breif history. basicly my sister brother father and grandmother all suffer with PRA. my mum had years and years of dr's telling her we were faking it and that she didnt love us enough and the pain we were suffering was an attempt to get attention. finally my sister got diagnosed by an american dr when we were living on an american airbase in germany. my brother got diagnosed by a dutch dr when he had blood poisioning aged 9 and the results showed some high RF. i never got diagnosed as a child as i was away at boarding school and somehow it got missed. fast forward nearly 20 years and my GP has ordered an anti CCP test and is finally taking me seriously.
anyway back to my son. hes 9 and for the last few years hes suffered with serious pains in his knees. mainly his right one but sometimes the other and occasionly both. hes crippled by it. his knee swells up and becomes very very red and hot to touch. i normally treat it with a ice pack or heated beanbag depending on his choice. sometimes we give paracetamol and if it gets really bad some ibuprofen but as hes asthmatic we go careful with the ibuprofen. finally this month his teacher took me to one side and told me he was suffering after swimming and sometimes he would just end up crying randomly because he knees hurt. so i took him to the gp. straight away she said given the family history she was refering him to a pead. she prescribed some painkillers for school and i felt we were getting somewhere.
last week we went for his paed appointment and i came out crying. basicly her opinion is that PRA is not hereditary, doesnt even run in families and she even implied that PRA is not even a real illness and is just like IBS, something you say when you cant label the problem as anything else. she told me that because i was making a fuss, he was saying it was worse. she inplied he was not doing enough exercise and that was way he was stiff in the knees. he told her he does fottbal once a week, swimming once a week, cricket twice a week, and agilty and balance club 3 times a week, and that he also loves to play dodge ball when he can. she said 'oh good keep that up' and then moved on as if he hadnt really done these sports weekly. she then told me to stop giving him painkillers, stop making a fuss and hes faking it. she asked me what exactely she wanted her to do, and i replied im not sure but i know hes in a lot of pain. i even told her my mothers experience to which she told me, well the dr's then might have been right, implying that me and my siblings are indeed faking it.
im at a loss as to what to do. she is writing to my GP to explain that in her opinion there is nothing wrong, and knowing my GP she will take this as gospel and will not help much more. how on earth do i get anyone to listen.
i understand that given my experiences as a child, i might be seeing things in my son that arent there. but i am not making this up, and my son is in agony when the pain comes. he is not the kind of child to make this up nor look for attention. in fact he hates it when i discuss it with his teachers. what do i do now?