I'm glad that your ds likes Singing Hands, dev - makes me feel all nostalgic for whn dd1 sang them in a repeat loop!
did you know they do classes for toddlers/pre-schoolers (and above!) too? we took dd1 along to an ad hoc session when she was about 5, as a reward for doing so well with her toilet training - she had a fab time and Tracy was really lovely with her - sang with her, encouraged her to come up and help, we even got a photo for dd1's beloved photo books 
might be worth trying for your ds, if he is interested in them? it's weird, as soon as i could identify which song dd1 was singing, it sounded really clear to me, but looking back, it was probably all gibberish with a healthy dose of parental over-estimation thrown in!
have you got any of the GIles Andrae books, like Rumble in the Jungle, Commotion in the Ocean, etc? dd1 loved these as the little rhyming ditties were a great length (short!) to catch her interest. they saved my bacon a few times too, as it was much easier to remember the shorter rhymes, and use them to engage/calm dd1 when out and about or away from the books, rather than eg the Gruffalo, or The Lion who wanted to Love - dd1 used to get so angry if I couldn;t remember all 42 pages word-perfect!
we used to read them to her with her on our lap, jiggling our knees along to the rhythm, so she got sensory stimulation too. it worked to catch her interest, and then she would make us read them again and again - literally all day long. I almost hesitate to recommend it, as it really did drive me very close to the edge - some days I would only be able to repeat the same story again and agin, on a continuous loop, for hours at a time. dd1 used ot also get stuck on particular words, as she learned them, so eg any mention of 'man' at one point, meant we had to stop whatever we were doing/talking about, and instantly sing 'there was a crooked man' to her - otherwise major meltdown ensued. this happened whenever she heard the word - on the tv, or radio, someone passing in the street, etc. it was a nightmare. other times, it happened when she learned a word from a book - whenever she heard the word, we had to pick up the story from that point on and recite it up to the end. that caused a problem with some of the longer books, I can tell you!
we also had the memorable phase where dd1 could only tolerate us talking if we did so to her tune of the moment - so we would spend all weekend conversing in rhyming couplets to the tune of Frere Jacques (or similar). Funny now, but my god it was awful at the time.
god, that all turned a bit depressing - sorry. in summary, I think what we did was go with what was currently fascinating dd1, and make sure she was totsally immersed in it - so she got it all on a loop for as long as she wanted. on the basis that at least that way we were sharing something - even if that something was more her using me as a means to an end.