"They agree with him in/on his approach to the problem."
Is it supposed to be "in" or on"?
Welcome to the forum, MYTakahashi.
"They agree with him on his approach to the problem."
"They agree with him about possible solutions."
"They agree with him in the present circumstances."
Rover
"They agree with him in/on these matters."
Is it supposed to be "in" or on"?
Both are possible. On means regarding, in means during. On is much more common.
What about: "They agree with him in/on his comments."?
Would it be "in" or on"?
Probably ON his comments, as they can't actually speak IN his comments; I can't see how in any case.