trembling hind legs

Question
We have a bull mastiff, which 9 months old. For some times now, his hinds legs tremble occasionally and he feels comfortable if some one holds his hind legs firmly. Earlier(may be when he was 3-4 month old) he had the tendency to fall while standing up (hind legs slipping). We thought, it may be due to the mosaic floor.

Answer
This is something that you should discuss with your veterinarian.  The cause could be anything from neurological (seizures) to orthopedic (hip dysplasia) or it might be something else entirely that may not be as serious.  I have no medical background, but I would definitely make an appointment to have this checked out.