I can understand the small frame too. Ben was only 5' 3" tall weighing about 6o lbs 2 1/2 years ago . Now 6' and weighs 140. I have always had to fight him to eat. some days he eats like a horse then goes a couple days not eating enough to keep a fly alive.
May sound like a silly thing but it helps keep Ben and his weight up Those drinks for seinors. he adds them to his reg daily schedual usually 2-3 a day.

After talking to mil hubby had same weight problem too as a kid. I met him 6'1' and 170 . after we got married 6'3" 180.after 15 year of marriage.
just before cancer stuff 6'5 200. now 10years after 6'5" and 235 just right.
Marry that boy soon and fatten him up
Sending many good vibes for you both.