I saw him (1)/a couple of times (2)/ since May(3). /No error (4)

Answer

1

Explanation

have seen will replace saw because -
Since (Prep.) : from a time in the past until a later past time, or until now, used with the Present Perfect or Past Perfect Tense.
Look at the examples given below:
She has been off work since Tuesday.
He has been working in a bank since leaving school. Hence, I have seen him is the right usage