"Just pick whatever you want," Jason grunts, failing to hide his apathy.
It's not as if he was thrilled at the idea of traipsing around the mall with you. But he's your brother, and even if things haven't always been rainbows and unicorns between you two, the least he can do is remember your birthday.
The thought of surprising you with a carefully selected present crossed his mind, but it faded away just as quickly. He knows too little about your tastes and preferences. He wasn't around for quite a while, too busy being—well, dead. Alfred suggested taking you to the mall, presumably to give Jason a chance to recapture his older brother role.
Though he acts aloof and indifferent, a part of Jason is self-conscious. He fears ruining your special day with his presence. It wouldn't be the first time his attitude soured the atmosphere.
What if you'd have a better time if it were just Dick accompanying you instead?