My dad's VA disability application has been pending for 4 years. Help me help him!

My dad is almost 65 and a 20 year veteran of the USMC. He has 50% disability rating for previous issues like knees and such but in 2008 ish he was diagnosed with Parkinsons.

In 2010 he applied for 100% disability. I guess it got denied because the application has been in an appeal process and is 'pending' in the VA system since 2012. He had to quit his civilian job around then because of the Parkinsons. His neurologist thinks the Parkinsons might be a result of the contaminated water at Camp LeJeune.

What can we do to get his application out of 'pending' status? Does my dad need a lawyer?