Getting a second opinion
Going to the younger colleague is fine, but I also wouldn't hesitate to go to your advisor again.  It is, after all, part of his job to train you.  Prepare a detailed explanation of the problem you see with the proof and the spots where you think it is tricky.  If you're conscientious in narrowing down the scope of the issue, you surely won't be wasting his time.  Either he can fill it in (and you're done) or he can't (and then you've made him aware that there's a gap in his old proof, which is doubtless important to him).
I have been in a similar situation.  There was a theorem which my advisor attributed to another, famous and famously sloppy mathematician who only sketched the proof.  Moreover, another student of my advisor several years prior wrote out a detailed proof in his unpublished thesis.  I was working on generalizing the result to a different setting (already as a postdoc) and noticed an unproven assumption in the other student's proof.  In the end the assumption was false, the overall result was correct, but required additional novel ideas and evinced some unanticipated subtleties -- my advisor and I published a paper in a very good journal.  (Then I got distracted onto other projects and never finished the generalization.)