If you had “mastered” the 2 and 4 seem and a change I doubt you would need another pitcher.
Few pitcher truly “master” more than 2 pitches anyway.
As for the gym there is a lot of stuff you can do, I don’t know how much will directly relate to velocity.
How tall are you? How much do you weigh? Have you worked out before? What sort of working out have you done?
Knowing where you are starting from is very important. Having a plan in place and sticking to it is just as important.
There are exercises that do not directly relate to pitching than they do any other activity but that you should probably be doing as part of lifting…deadlift, squats, lunges (forward and back), back raises ect. Long list of potential lifts you can do.
There are exercises that are not pitching but that more directly relate to the activity. Longtoss, skaters, med ball tosses etc
Then there is pitching…throwing from a mound, flat grounds, mechanics work etc
So, first thing is to do an honest inventory of where you are. Are you weak? Is so that is fine, it is something to work on. Do you need to lose or gain weight? Great, another thing to get better with.
A good program with have a mix of all three elements as well as plenty of consideration for recovery time. In my personal opinion a program worth its weight is based first in arm care for a thrower.
Mr. Ellis’s Tuff Cuff book is a great resource that covers a ton of this material in an easy to follow way and is very reasonably priced.
If you are going to get serious with it I would try to avoid “bro” workouts at the gym, or working out with the football players from school ect.
Set reasonable short term goals and write them down and track your results. If you squat 100 lbs and you want to gain 50lbs on your squat give yourself a good time frame and work hard to get there. If you ware currently throwing 75 mph saying you want to throw 95 mph is like saying you want to walk to the moon. Saying you want to gain 5 mph while improving your curveball by next season, that is a real goal. You may very well exceed it, but, it is reasonable enough to mean something.
Finding another guy who is as dedicated as you to throw with consistently, to hit the gym and actually work hard, not just chat and mess with the music on your phone is key. Write down your workout results and your calorie/protein intake if you are trying to lose or gain weight.
In my opinion the reason the coach is asking to you add a curve and another breaking pitch…so with the change, 3 off speed pitches, is he doesn’t think you are going to add velocity. Answer the questions above, work hard and prove him wrong.