1.0 He had to give up the idea of studying abroad for financial reasons. Short answer: Although it doesnt "have to be" plural the standard collocation here is "financial reasons." Perhaps the reason is that there are a number of possible financial reasons. For example, not having enough savings, not being able to come up with funding/financing, not being able to secure a part-time job or scholarship, etc. Thus the collocation is generally pluralized and leaves it up to the listener (or reader) to seek the concrete details of the actual reason(s).
1.1 He had to give up the idea of studying abroud for the financial reason(s). This is a possiblity if the context were to allow it and it could be either plural or singular. For example, it would be better if it read something like "...for the financial reason(s) that we mentioned earlier." In either case it would be either a set of reasons or a singular reason that both the speaker and listener are familiar (share the same background knowledge about).
1.2 He had to give up the idea of studying abroad for a financial reason. Like 1.1 this too is possible. Here the speaker would be familiar with the specific reason and would assume that his listener is not familiar with it (thus the use of the indefinite article).
I think we (native speakers) may have a preference for "financial reasons" as it is a little vague and helps maintain a sense of privacy for the person being discussed. This is important because matters of personal finance are probably best considered taboo topics.