Do any contemporary Orthodox poskim discuss the matter? What potential problems could arise from attending, or from not attending, if the Bar Mitzvah boy is a close relative?
All contemporary Orthodox poskim are in view that the reform movement, since denying commitment to the Tora is in fact like "raising it's hand against the Tora" (Meirim Yad Be-Toras Israel), hence it is forbidden to attend. The Gmara (Shabes, page 116) states that you are not allowed to enter their synagogue even when they are not davenning.
Most poiskim advise on doing your best to avoid conflict, each case should be considered by itslef. One could pay a private visit, before and\or after the ceremony, bring a nice gift, explain, if relevant why he could not attend or alternatively, find a "good excuse" for not attending, such that will minimize the inconvenience.
One view that is less strict is the Maharam Shik on Orach Chaim (71) who says that in great need there might be a Heter to enter such a synagogue (not during a prayer of course), and he states some conditions to that, e.g. when several enter together etc.
It is of the utmost importance to try our best to keep peace and good relations with all fellow jews, even those who have strayed off the path of the Tora.
If you read Hebrew, you can see more details in the following Q&A