What Are The Chances Of STD Transmission Through Physical Touch?

Two days ago I met another man and we were masturbating in front of each other. At one point he reached out and stroked me for a few seconds before going back to himself. I did not see any sores on his genitals, and he claimed to be STD free. I now have what looks like a pimple on my scrotum. I regularly shave myself, and sometimes I get small whiteheads after shaving along with mild razor burn. So it might be from that. My question is this, what are my chances of contracting an STI (Herpes comes to mind) from this encounter.