I should think he is far less likely to be embarrassed than you are.
It seems to me that it is just one example of a wider phenomenon —
Anxiety about getting an erection in a given situation + that situation = an erection.
In my case, I can think of at least two examples. I always knew that it was vital for me not to get an erection in swimming trunks. So, for a while as a young guy, the simple act of pulling swim gear on would pretty well always cause a rapid erection, which wouldn't go down until I got into the water.
Much worse. Way further back, at school, there was one particular boy who often drew attention to my penis in the showers or in the changing room ... if he thought that someone else hadn't seen it before. Sometimes just by pointing, more often by some remark which he thought I couldn't hear. I got very nervous, very keen to avoid him, even more so because I got so self-concious that he simply had to be within ten feet of me for at least the beginnings of an erection to click in, sometimes the full blown article. Which just made him stare and point even more. I got to be the quickest boy into and out of the showers, and often pulled on my clothes any old how when still not properly dry. He might have begun to think that I fancied him, which was far from the case. | 
