The fatigue comes from using muscles in ways that he doesn't typically use them. That should lessen as he does the exercises more.
I can't explain the strength difference unless something else is going on in the arm. Do the lifts get painful or does he just run out of strength?
My son is a RHP just coming off of a couple months of physical therapy due to tendonitis. Working with him in the gym I have not noticed any difference in strength from one side to the other.
When you talk about the lifts with 20 lbs - are you talking about just normal curls?