How can I encourage a teenage brother to not keep giving up his bodybuilding program?

My bro is 15 and he will workout for like 3 weeks and drop off for 3 weeks then start again...
He doesn't stay consistent. I also think his lack of discipline might hurt him in other areas of life.
There was a point when, in spite of not being particularly athletic, he was gaining muscle pretty well, and said he'd even consider one day entering a bodybuilding competition (natural only, of course).