I frequently hear sentences constructed in this way.

Problem:
A verb’s past participle appears on its own where the verb’s past-tense form should be placed.

Explanation:
This problem is the complement to what I posted yesterday.

In other words, there seems to be a tendency among some American English speakers to mix the past participle with the past tense.

The past participle of the verb “see” is “seen”, and “seen” requires an auxiliary verb — in the present or past tense — to form a perfect tense.

Otherwise, the past tense should be used in place of “seen”.

This gives us three solutions, depending on the intent of the speaker.

Solutions:
“I saw him.”
OR
“I have seen him.”
OR
“I had seen him.”