Hi Puna!
Jeans (Levi's 501 for instance) or dockers, and a shirt, a polo or even a t-shirt. Add Nike tennis shoes and he will appear a spaniard
He will be fine almost with anything save:
- Plain white socks with normal shoes (they could be ok with tennis)
- A cap (no way!)
- Sandals (if worn along with white socks he would instantly be labeled as an unfashionable "guiri")
- Hip-hop or rap style of wear
For the Camino de Santiago he better takes shorts, and other comfortable stuff (white t-shirts, because it's warm weather usually), good trecking shoes (or boots or sport shoes), a jumper and a good coat (just in case it rains or it is colder than expected).
For entering the Santiago's Cathedral and churches along the Camino it is ok just the above mentioned. Try to avoid very excentric dressing. The rest is allowed (even shorts and t-shirts, it is assumed you are walking through the Camino).
Hope this helps.
Fernando