Keep in mind that most decent V programs have the guy with the strongest arm, athleticism and fielding ability playing SS, so don't take it too hard. It's great that you are listening to what he is saying regarding things you need to work on and taking action.
Check out the J-bands and throwing program here...
Also, when you take ground ball reps, be sure to take several deep in the hole and work on making that long throw strongly. And be open to earning whatever position you have a shot at winning. If the current V SS is returning, that may mean better opportunities exist at another position.
I will add that it is also very important that you can make all the throws consistently. As a HS coach, there have been many years where we looked first at guys with strong arms but ended up putting someone there who we could count on to make all the plays that should be made.
Good things tend to happen to those who work the hardest!