It IS possible to get a harder erection, to an extent.

I remember one time I was so horny that I couldn't put on pants. I just couldn't. Normally, if I get fully hard, if I put on pants, it just hurts a lot, but I can normally bare (Wait, which spelling do I use there? Bare or bear?) it. This time, it simply hurt too much. I found the baggiest pair of pants I had, and even when I put them on, it felt incredibly tight, and the outline of my head was so clear you could trace it with a pencil, even through the boxers and pants. I'm not sure why I had to put on the pants, but anyway...

When my cock was out, my it was most definitely harder, and it was straighter and aimed up more at the base. The tip of my cock was on the same level as where it started off, whereas it's usually lower due to the downward curve. The veins on my cock were also a lot clearer. I didn't have the means to measure, but I wouldn't be surprised if my cock WAS bigger, as hard as it was.

I'm not sure if this also occurs during edging, but yes, an erection like that IS possible.

You were sure to tell him like it felt he was getting bigger inside you, right? If it were me, my GOD that would be such a turn on!