tā shàngbān de shíhou yídìng yào dài yǎnjìng, bú dài jiù kàn bù qīngchu diànnǎo.

Literally, 上班的时候 means the time when [he] is at work / going to work.

The pattern is:

• verb / verb phrase + 的时候 = when ... / while ... / at the time of ...

So:

• 上班 = to go to work / to be at work
• 上班的时候 = when (he) is working / when (he) is at work

Here, 的 links the action 上班 to the noun 时候 (time, moment). It is very common in this structure.

Examples:

• 吃饭的时候 = when eating
• 下雨的时候 = when it rains
• 我小的时候 = when I was little

It can sometimes be understood either way, because 上班 can refer to the general activity of working or going to work.

In this sentence, though, while he is at work is the more natural reading, because the next part mentions seeing the 电脑 clearly. That strongly suggests he needs glasses during work, especially when using a computer.

So in context, 他上班的时候 is best understood as:

• when he is at work
• or while working