Is a son's taking a monthly sum from his father in exchange for completing a strenuous task previously agreed upon considered a form of forbidden gambling? If so, how can it be rectified?

There is no harm in benefiting from the rent given by the father after the requested work is completed. This is a permissible ja'alah (reward for a specific task) and not a prohibited wager. It is valid to stipulate the completion of the work within a specific timeframe, such that the worker is not entitled to the reward after it, unless the period has ended and the result is close to being achieved, in which case it is automatically extended.