My 13 year old DS1 is 5'2" and 125-130 lbs. He is wearing men's small shirts and 28x30 pants which is on the small end of the men's size scale.
So I'd say that your grandson is at the transition point - size 16 seems fairly close to men's small. Depends on his build too. My DS1 is of stocky build, so he needed to move to men's for shoulder width and thigh width (he's a hockey player and it's thighs are enormous). If he were lanky or really slim, I'd keep going with the boys sizing. Boys sized pants don't work for my son anymore.
Karen
-----------
Mom to 2 hockey-playing, Lego-loving boys DS1 2003 & DS2 2005