my 21-month old isn't talking yet?

My baby is 21 months old and we haven't heard him say a single word yet. Sometimes it seems he understands us, because he will put things back up when we tell him to put away something he's thrown on the floor or forgotten about, and he'll clap at the ends of songs such as "If you're happy and you know it" and the "Itsy bitsy Spider", etc. But my concern is that he does all of this out of imitation of our actions, like in the sense that if we get onto him about leaving something on the floor, i'm not so sure that he puts it away because he inherently understands what we're saying, or if he's complying out of a sense of repetition because he's seen us act it out. If that makes sense.

I don't yet have a strong feeling that he understands, because when i try to read books to him he's more interested in flipping pages and never focuses on the colors on the page or the objects or numbers or whatever it may be. We're forever trying to get him to say words, repeating stuff over and over, but he makes no effort to say them back and i'm worried that something may be wrong. He voices frustration by crying, and hunger and all of that, so we've been getting by on distinguishing his different cries. He babbles a LOT, as though he's giving speeches to objects, and he'll give like babble commentary over objects, but should i worry? Or does he sound like a late talker?

He's intelligent, and can imitate anything after seeing it once, like putting keys in locks, figuring out electronic stuff (plugging stuff here and there or swiping on cell phones), putting tops back on bottles, etc; he can feed himself, almost put his clothes on (the motions, anyway), but it's mainly just the speaking that lacks.

Most recently i tried to get him to say "keys" and he said the "eez" part, but didn't pronounce the "K". So at the very least, i think he's solid on vowel sounds, just not consonants? But i wanted to know if any of your experiences with children matched this, because i'd like to rule out hearing problems as the source of delay.

If you can, could you give me advice on how to test at home to see if he has hearing problems? I've tried to see if he recognizes songs in the absence of me miming them out, and he can, but is this a thorough enough test? thanks!