National rules are that you are only allowed to apply Early Decision to one school, and if you get in, you are committed to going there. This is not the same as Early Action, or rolling decision.
Many High Academic schools have Early Decision, and the athletics departments are allowed to support athletes only in the ED phase. So, a coach "commits" to giving you one of his supported slots (the key term) in ED, and you "commit" to applying ED.
Note that he cannot 100% guarantee that you will get in, you should ask him what the chances of admission are with that level of support.
This has greatest value at schools where the acceptance rate is so low that you otherwise might not be accepted, or if your grades/test scores are lower than the school's average. The school commits to admitting the coach's 4 or 6 or whatever choices in the ED round, and then they are less likely to admit any baseball players in the Regular Decision round.
And yes, if your second choice also requires ED, then you have to make your decision by late September. That means visiting schools in September. It's a very precise dance. Coaches need you to make a decision, because if you decide to go elsewhere, he will offer that slot to someone else.
If you don't need the coach's support with admissions, then you don't need to apply ED. He might still ask it because it is a sign of commitment to that school. If you are unsure about things like financial aid, most schools will give you a financial aid estimate before you apply - that is definitely something you can ask the coach about.