When to make up all the missed obligatory prayers

He [was] ill and underwent surgery, and [during this time] he missed a number of obligatory prayers; so should he pray them all together once he is better, or should he pray each obligatory prayer at its time, such as ‘asr at the time of ‘asr, and zhuhr at the time of zhuhr, and maghrib at the time of maghrib…and so on?