Once you know, you are kinda obligated to. Like you could choose to ignore but then you would have to tell her that I haven't gotten you anything although I know it's your birthday (plus he doesn't know she is preparing stuff at home for the two anyway). Or you could just choose to be completely oblivious about it and several years down the line, Akari is gonna hang out with her best friends and she might slip: "Oh I accidentally slipped your birthday to Jirou. Did he not get you anything that day? That's pathetic". Of course it's probably not gonna happen but I'm just pointing out the negativity it might cause the relationship.
Plus, it's gonna be more impressive if he knew without her telling him directly and he still got her something. Tho you could argue they are pretty much dating at this point so there's no point