What causes low sperm count?

Just curious hubby thinks there's something wrong with him when I keep on telling him no. He took antidepressants before he met me for 6 years I believe and that's why he thinks we haven't concieved yet. He also had chlamydia and thinks that's why he's infertile. I asked my ob gyn If guys can still have kids after chlamydia he said yes they can become fertile again. We been ttc for 8 months and nothing so as of now I'm not trying to stress just waiting to see what happens