My daughter (34 months) went to the library's toddler hour a little less regularly than you do (we have to drive a ways to get into town and it was not always happening, but I tried to make it as often as possible because I know she liked it.) I took her to "Rhyme Time for Tots" when she was an infant/young toddler. That group was 4 months-18 months, and it was perfect for her because she didn't walk until 16 months, anyway. It was a small group, all moms and babies and lots of lap bounces, fingerplay, and rhymes. A couple of short books read by the librarian. A circle dance (where we'd carry the babies and sing) and just a nice time.
The toddler hour (18 months to 3 years old) is more physical with the kids doing a lot of actions themselves, turning motions and songs, like "Wheels on the Bus." Lots of rhymes and ring around the rosie and several longer (but not long!) books. I knew my daughter knew the songs because she would sing them at home, but she mostly just sat on my lap or right in front of me, and watched. She watches the librarian intently, but never sang or participated. For awhile, I still had to carry her if I wanted her to participate in "ring around the rosie," and she finally started holding a hand and walking but never would fall down. Etc.
She also went to the preschool story hour just once (it's for older children, she was newly 2) when the librarian mentioned she was having a dental hygienist as guest and it might be good for reinforcing toothbrushing habits. That time was more structured, with the story, the guest, and a craft.
We have not gone as regularly this year (only once this fall!) because she's in a preschool program and on our days off I don't do a lot of trips to town. But that one time, she did sing and do motions. (Which she's been doing at school, after a lengthy period of just observing, then just doing motions, and finally singing+motions.) She also fell down during "ring around the rosie." Astonishing!! She has always sung in detail (all the words) at home, though, so we knew she was watching and picking it up.
The fact that your son sings at home and reports on the craft to his daddy sounds like he's taking it all in, enjoying it, and participating/processing on his own terms and at his own pace.
Though I delight in my daughter's seriousness and careful observation, and am happy that her reserve doesn't seem to contain fear or misery (I was painfully shy, with true pain involved), I definitely have noted the contrast to other young children. And as accepting of her as I am, I have questioned if she "should" be more of a joiner. It's part of paying close attention, noting differences, and being a little sensitive/protective, I think. I try not to suggest that I think she should be any different than she is, which means (among other things) that my husband and I don't discuss much of this type of thing in front of her (even just simply making observations about differences or communicating what she was like at an event.)
The irony is that she's quiet and self-directed at pre-school, but her teachers say she is a very central figure in the group. She's important to several children in her class, and they often invite her to join them, want her to pull them in the wagon or paint at the table with them, or whatever. They also take her word for things, if she says it's her turn to play with something right now or whatever. Her teachers say she calmly and quietly lays down the law, and her classmates seldom take issue with her take on things.