Panosteitis in big breeds

I have a 8 month old mastiff who's been having lots of problems with his legs the past month or so. We've been to the vet multiple times and she can find nothing physically wrong and has put it down to growing pains and has gave painkillers for him. She said he is growing so quickly and his bones are finding it hard to keep up. Has anyone dealt with this before and can offer any advice? He is in so much pain and, at times, is completely lame and can't move. After doing some research I've seen that this can continue until 1 1/2 - 2 years old and I just feel so horrible for him. He can't go on walks or play and just lies around all day crying, which is completely unlike the active pup he was before this started. Is there anything else I can do for him, all suggestions are welcome. Pic of my lovely boy