Answer:
Handle his money wisely.
Step-by-step explanation:
'To watch his money' is an idiom.
An idiom is a phrase or a group of words used to mean something else. An idiom can not be understood in a literal sense but to understand it applied meaning needs to be understood.
For example, 'on cloud nine' is an idiom which means to convey the meaning of extreme happiness.
In the same manner, 'to watch his money' is an idiom that means 'to handle the money wisely.'
So, the use of this phrase in the question means 'to handle the money wisely.'