Here is a link to a set of Core exercises from a pair of college coaches that I trust - Coach Micheal Cano and Coach Brian Neal... these are TOUGH and meant for college athletes. Modify them as required but give them a try - start with Day 1.
https://www.youtube.com/
If you do try them, let me know how you did and what you think of the program.
Coach Bob
15u@kanatabaseball.com