The numbers seem alarming. I know guys look at hot girls and collect girls' photos, but to collect in thousands seems abnormal. If you give him a few more years, he'll probably have more than 10,000 girls collected.

He either has too much time and energy on hand (i.e. he's bored and unproductive in life), or he is rather stressed and uses lust and porn to release tension. This may indicate a lack of healthy hobbies and supportive social circles.