My 13 year old son has a rash on his butt that really stinks....I mean I can smell him walking by. What can help?

If it smells bad, there is likely an infection, or at least bacteria living on the rash. Often times, what makes things smell bad (for example, your under arms) is bacteria living on the skin (so not the actual sweat from your underarms, but the bacteria living on there).
No one on Yahoo answers can definitively help your child, and you shouldn't trust any medical advice given on here, especially when it could be potentially dangerous. Get him to a doctor, make an appointment with his pediatrician ASAP.
Even if it turns out be nothing serious, it's better safe than sorry.

Stay upwind.

baby powder

Fa doctors visit.

a doctor. Sounds like a yeast type infection or bacterial infection. he needs to see a doctor to have ti diagnosed