Agree, if I were him, I would look on how Andy Murray bulked up in 2008. I remembered that Murray was "skinny" in 2007 and he got that wrist injury in Hamburg. Next thing I know, he put on some muscles and played a lot better in 2008.
Now, I don't expect Sinner to fully bulked up like Murray (probably not advisable), I do expect him to put some type of muscles and like you say, more strengthening especially the core muscles.
I just looked up Sinner's height and weight and he was listed at 6'2 168 lbs (76.2 kgs). In my opinion, he needs to put at least 5 to 7 lbs of muscles especially in his core and lower legs to be fully effective.