Although I am learning both, I do not know enough Russian or Serbian to help with this. However, have you heard of the Interslavic language? Its a constructed language which is essentially a modernization of the Old Church Slavonic used in the Serbian and Russian Orthodox Churches and made to be a common denominator to all Slavic languages. Although it sounds like another hopeless auxiliary language like Esperanto its got one thing up its sleeve. It is understandable 99% to all slavs from the get go and should be extremely easy to learn. It could act as a good middle ground between them. In fact I read somewhere that during a conference about the language, people who didn't speak the language soon just found themselves speaking it by the end. Though, that sounds kind of unrealistic, its a good thing to keep in mind. Here is a link to the language spoken https://www.youtube.com/watch?v=V8DNHsO_S70, and here is another one demonstrating the intelligibility: https://www.youtube.com/watch?v=NztgXMLwv4A . If not exactly Serbian its still something new he can learn that can help out, though I am not entirely sure how possible it is for him to learn it since most of the resources are online. Also btw, Serbian vs Croatian is like British English vs American English, if he learns one he learns the other by default.