I think it's correct what your son wrote, but I agree with the teacher that it sounds better to say "You can only see.."

Strictly speaking, both sentences are correct:
You can only see ten houses - could mean ten houses and nothing else
You can see only ten houses - means no more than ten houses, and whatever else there might be. The emphasis is on the number, though.

"You can see only 10 houses" sounds stilted.

"the position of only in standard spoken English is not fixed, since ambiguity is avoided through sentence stress; in casual prose that keeps close to the rhythms of speech only is often placed where it would be in speech; and in edited and more formal prose only tends to be placed immediately before the word or words it modifies."
http://www.merriam-webster.com/dictionary/onl...

The teacher was following the custom of "casual speech." Your son wrote "more formal prose."

Theoretically, both are correct.