This is kind of the McVay way. The scheme takes a while to master, and you are expected to block. Jefferson didn't see the field much last year either and he had size and pedigree.
One thing I hope the Rams do is stipulate a weight for him to hit by mini camp or training camp. Teams do this with too-heavy lineman sometimes. Here it would be the reverse. If he's truly 155 (is he?) then he needs to be maybe 165 by mini camp and 170 by training camp. With the strength training, nutritionists , etc at his disposal from the team - adding 10 or 15 pounds of good body weight shouldnt be an issue seeing how young he is.
His offseason needs to be 6 days a week of weights and calorie monitoring to have him at a surplus. What did he do after being drafted - run, play basketball? He obviously didn't spend it trying to add to his slender frame