I was reading over some of my supervisor's old papers and I have found one from 7 years ago in which part of the analysis is fundamentally flawed which effectively undermines half of the conclusions. The issue is in a paper in a subfield of a subfield so it's unlikely that any other people will find this mistake.
My question is what should I do. On one hand, I feel like I have moral obligation to let him know so that the issue can be resolved. On the other hand, I feel uncomfortable bringing it up as I don't want it to have any negative repercussions on our working relationship.
What should I do in this scenario?
Edit: This question is different from Should I warn my professor about some errors that I've found in his paper? as in that case it was a paper in advanced access (e.g. one that was worked on recently), while in this case it is a 7 year old paper.