Help needed with my dog please

Hi Vets, my dog is almost 6. October til December last year he started with night time anxiety every night which I have since learnt was actually pain. He started on pregabalin and meloxicam and the night time anxiety stopped. Since October he's gone from being an active dog to a very lazy dog and stopped wanting to play with my older dog which they used to play all day everyday and not wanting to play fetch. He sees a canine remedial massage person who told me his stance is very wide, his right back paw points outwards instead of inwards, his right leg leans inwards when walking, he leans forwards putting most his body weight on his front legs, his back right leg seems worse than his left. When he pees he cocks his back right leg because the occasional times he cocks left leg he falls over. His back right leg muscles are always very tight when he gets his massages, the left tight but not as tight as his right leg, his front legs the muscles are sore especially front left leg, which I was told it's because they compensate for his back legs. His back along his spine is very sore too when he gets his massages. His massages help a little. When he gets up from laying down or lays down I hear cracking sounds. He started preferring to sleep on floor instead of his bed and doesn't jump up on couch much any more. He started scrapping his back toe nails occasionally on walks which has now become more frequent. He has never really sat he either stands or lays down but occasionally when he does sit he sits off to the side and doesn't sit for long. When he lays down he crosses his back feet. He sploots sometimes but he possibly learnt this from my staffy. The canine remedial masseuse he sees who is also a qualified vet nurse thought it was possibly hip dysplasia or bone spurs but he recently had an xray of his spine and hips which came back normal. No arthritis, no hip dysplasia and his muscle mass is outstanding. His vet said we could do mri to check his spinal cord but she doesn't believe it will show anything given how good his xray was but I'm at a loss at to what could be going on and was wondering if you would have any insight please? Thank you in advance

Hello and thank you for this interesting case.   This sounds to me as though it might be neurological and I wonder whether a full neurological examination has been carried out; it would be something to ask your vet about.  Not all neurological problems are spinal.   Another group of diseases that might be relevant are muscular diseases.  You don't mention your dogs' breed but sometimes muscular problems are specific to certain ones.  I wonder whether further investigation or referral may be relevant here.  Sometimes dogs do compensate for neurological lesions for example, with compensatory muscle mass development - obviously Ive never seen your dog, but dont let good muscle mass discourage you from investigating.