Mazel tov!
Our boys are aprox 2.5 year apart, so we’ve been thru this before. Lots of love, and private time with sib # 1 is a must. Even if its only 10 minutes a day. And understand how he feels: Imagine being told by your spouse, that since you value him / her so much, you’ve decided to get another one! And not to worry, you’re not being replaced, this is just a new addition to the family. Oh, and you’ll need to start sharing your room, toys, time and attention (that, till now, was 100% yours) with this new arrival.
How would that fly with you? In many ways, this is what we are telling sib # 1. Its not until the later years they understand how much fun a partner can be (and no, I’m not advocating for a 2nd spouse!)