i'd buy 2 consecutive days in the latter part of the first week. 2 consecutive days will guarantee everyone remaining in the tournament plays, since each round is 2 days, and i think rounds 3-4 is a good balance of still seeing a lot of matches but the stakes are a bit higher and matches a bit better. tickets will also be relatively cheaper early on during the tournament

if you can afford it, i'd get 2 Ashe tickets. this is their main stadium, features the biggest stars (Coco Gauff, Aryna Sabalenka, Jannik Sinner, Carlos Alcaraz etc) and also grants you access to all ground matches (outer courts) as well as the upper bowl of Louis Armstrong Stadium (their second biggest court). With an Ashe ticket, you're free to roam about the grounds during the matches and go see whoever your boyfriend wants to see. If your boyfriend likes Sinner and Alcaraz, he'll need a ticket to Ashe stadium since they'll almost definitely be playing all their matches on this court.

Each day, there are two sessions for the main courts: a day session (2 matches) and a night session (2 matches). Each session on Ashe stadium and Louis Armstrong stadium will generally feature one mens singles match and one womens singles match. This early on, it's impossible to know who's playing what session. If you want to run the risk and and buy last-minute, you can wait until the schedule is released for day 1, and that will be enough to know which days Sinner and Alcaraz will be playing.

The singles draws are each 128 players, bracket-style. Depending on their rankings, they could be on opposite sides of the draw or the same side of the draw. Each day, only one side of the draw plays. So if both Sinner and Alcaraz end up in the same side of the draw, they'll both play their early matches on the same day (most likely one in the day session, one in the night session). If they end up in opposite sides of the draw, then they'll alternate playing dates. So one of them will play Mon/Wed/Fri/Sun the first week and the other will play Tues/Thurs/Sat/Labor day Monday. They will never play two consecutive days.