If he complains, add to his burden.-إن ضج فزده وقراً

It is also said, if he drags his feet, add to his weight. This originated with camels, then became a proverb, because a man may undertake a task but not manage it; rather, he becomes weary of it and asks for it to be lightened, only to be given more. As it is said, "Adding more burdens brings you closer to achieving your goal." And similar to this...

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The closest English equivalents are:

Adding insult to injury

“To make matters worse.”

This is the closest and most common.

To add fuel to the fire

“To make the fire burn even brighter.”

Pile more on his shoulders

“To increase the burden on his shoulders.”

Kick a man when he’s down

“To strike a man when he’s down.”

This is used when the intention is to be harsh on the weak or tired.

The closest expression that captures the spirit of the proverb is:

If he complains about the load, give him more to carry.

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 ويروى، أن جرجر فزده ثقلاً، أصل هذا في الإبل، ثم صار مثلاً، لأن تكلف الرجل الحاجة فلا يضبطها، بل يضجر منها فيطلب أن تخفف عنه، فتزيده أخرى، كما يقال: زيادة الأبرام تدنيك من نيل المرام. ومثله