Update - we did go see the dr. He also gets a bleeding nose really often and that's another reason we were going. My dh did get a very basic measure of my son's penis (no measuring tape, no comments). According to the linked chart, it is small. However, the dr gave him a very cursory look (our concern written down in advance in her chart, so nothing would need to be said out loud or pointed out in front of ds). She did not mention the tight foreskin (all my fears for nothing, thankfully) and did not attempt any manipulation of his penis at all. She later told dh that he is fine, and that 'there would be no treatment for it anyway'.
I wanted to say thanks for all of your comments. I found the comments from the other parents mentioning the different sizes of their boys' 'parts' helped to allay my fears. I am not worrying about this any longer.