I would be speculating to say a person weighing X and dropping to Y without changing diet is health related .
When turning 50 several changed in group I know .
1) One guy developed peanut allergies unexplained
2) One has a serious issue to gluten . Any amount = very sick .
3) Lactose issues as you stated .
4) Sleep apnea several guys .
5) Low test .

I would add tablespoon here and there of safflower oil . That is good fat & calories .
You can add to oatmeal , dip meats in it and drizzle over veggies. . Little taste to it
I would check for IBS and colitis & crons with doctor . Hopefully at 50 he has insurance for colan check .
Eating is one thing , utilizing the calories is concern .

Keep us posted of anything .
DWBO