Hey Joanne!
Great news that he is doing well in the lumbar spine. Regarding the triceps weakness:
C6- C7 for sure. I would use a 25mm needle at that level. Some evidence that lung tissue can sneak up to that level and if you are too wide you could have an accident for sure. So be sure to be behind transverse process, aim for lamina, and use a 25 mm needle.
You could also follow down radial nerve path in the tricep on the humerus- or at least hit tricep heads and MT junction.
Hope that helps!
Keep us posted!
Sue