Depends on which maoyi you mean. Why don't you write in Chinese?
贸易 commercial trade
毛衣 (woollen) pullover
So it is either:
1. 他要讲关于贸易方面的问题 。He wants to talk about trade related issues. (As Zhang the Great said above [love that name])
2. 他要讲关于毛衣方面的问题。 He wants to talk about problems associated with woollen pullovers. (Maybe allergies, discomfort, the need to wash carefully)
I'd choose 1. but 2. is a possibility.