15 year old brother rarely has "bm" and found out he is using pads in his underwear, which are covered in feces, to help with mess. What's going on?

Chronic Retention. Some kids have a slower emptying cycle than others &/or begin a pattern of skipping stools.Early:avoiding discomfort of emptying; school age: avoiding school bathrooms or class embarasement:older; not being able to empty effectively because of balooning of the sigmoid colon.New stool slurries around old compacted poo & leaks.Needs confirmatory xrays, a great wash out & a year long rehab program.
Needs to see MD. Coould be have "overflow" diarrhea from impacted stool in the rectum. Definitely not normal. He needs to see a doc, get examined.