The question is whether he's coming to visit us next month.
Literal
Problem [topic-は] next-month he [subj-が] us [obj-を] come-to-visit [whether-かどうか] [copula-だ].
Demonstrates 問題は~かどうか with a different embedded question. 訪ねてくる ('come visiting,' 訪ねる+くる) uses the motion-toward-speaker ~てくる auxiliary. The subject of the embedded clause is 彼, marked with が (typical for embedded/subordinate).