One of the best ways to determine whether or not said ch1ld is yours in this testing is to do the necessary testing and swaps. However, your ex partner said that your cock was "too small to ever be able to reproduce". I'm not one to get all wrapped up in the drama of romance, and it does seem a bit silly... That's figure this out though.
Show me what you have down there, and I can get a sperm sample from you. We can see if you actually CAN have any of your own to begin with, yeah? I won't judge. I just need that sample.
SURELY your cock won't be THAT small, right? Go ahead and do this right now. Don't mind me. I'm sure it isn't anything I haven't seen already... If it is a freak show though, I'll have to see what I can do