my father ... under weight ...?

My dad is 6'2", weighs 159.8 and is 56 yrs old.  He works out with weights 3 days a week and does cardio 3 days a week.  I looked at the bmi and his bmi is 20.5 which i guess is normal. 

My mom is extermely concerned about his weight.  In a week he will be going hunting and he is known to lose up to 3 to 5 more pounds. Is 159.8 okay for my dad or does he need ot gain weight?  If he needs to gain weight do you guys have any ideas how he can get more weight in a healthy way?  Im wondering if he is losing muscle mass even though he is working out.