My son dealt with the reverse problem. He wanted to play three sports. While he knew baseball would be the future he loves ball just as much. His best sport was soccer.
The baseball coach didn't mind him playing other sports as long as he got in his off season baseball work. Freshman year my son attended the two 5:15am baseball workouts played basketball in the winter.
Despite being the starting point guard on the freshman team he was cut the next year. The reason was not playing summer league basketball and missing all the "optional" fall workouts. There were parents of returning varsity players who told me he should have made varsity soph year. In the fall he was busy with soccer and travel fall ball. He spent free individual time practicing dribbling and shooting.
The soccer coach had a reputation of cutting anyone who didn't play elite soccer in the summer. But he couldn't pass on my son. He had him pegged as the goalie back to when he was ten years old. He would piss of the coach by smiling and saying, "It's no problem not playing travel soccer. Outside the 18 is like playing short. Inside the 18 is like playing third."
My son went through high school playing soccer, baseball, intramural basketball and travel baseball. Getting cut from basketball allowed him to focus in the winter on baseball. Instead of running off weight on the basketball court he added 25 with a protein diet and lifting. The next winter a hitting and pitching coach elevated his game another level. This wouldn't have been possible playing basketball.
My advice is if a kid focused on baseball is going to play two sports make it fall and spring. It leaves the winter to prepare for the upcoming baseball season.