I think they probably married a time after Kohane was trying on clothes, it did seem for a ceremony or something, in the OVA is mention the time in which they married, its around the request from the professor.
I don't think we see Shizuka's son, we see his grandson instead, to shows how MUCH time has passed on, it was around the time when Watanuki was having the butterfly's dream, my heart SUNK when I realized that wasn't Shizuka :(
Also, worth mentioning, Shizuka and Kohane married in order to protect their most important person, the one they love, and to create a bloodline that will forever stay with Watanuki as he won't age and will have a long long long life.
You won't believe this, I'm so dumb I didn't notice Shizuka and Kohane-chan married and the 100+ years passing stuff and tought Sayaka was Shizuka and I even read all xxxHolic Reí chapters still thinking he was Shizuka, and now I'm so shocked I'm honestly wondering if I'm dreaming... Can someone (if it's not a bother) please tell me when did Shizuka stop showing and was "replaced" by Sayaka? And when did it show(?) that he and Kohane-chan got together? I'm so confused help me please. ;-;
P.s.: Despite me being this dumb, I REALLY loved this series. Definitely one of my favourites.
P.s.2: Sorry if my english's bad, if something I say is not understandable please tell me.