I would try doing things in more of a play setting. Instead of asking him where his eyes are, tell him you are going to "steal" his eyes. If He hides them from you, you know he knows what they are. If not, well then "steal" his body parts, and you can make a big game out of that. he can also steal yours.
He may also know more than he lets on. My DD LOVES to play ignorant... I can ask her to count with me, and I will get "13255371..." Etc... Yet when she is in her room on her own, I have heard her count to 20 without a mistake. Same with the alphabet... If I try to get her to recite it with me, she "can't" but I have heard her sing it entirely.
My theory (and her speech therapist's, because she is speech delayed..) is that she purposely pretends she doesn't know, either to get a rise out of me (come on.... you know this silly!) or to get me to play with her more trying to teach her.
I will say to press the issue at the next checkup. At my DD's 18 month, I mentioned that I didn't think she was speaking well enough. They blew me off,and at her next checkup she was even further behind. Up until they are three years old, you can get free help through early intervention. Once they hit three, you have to go through the school district or a private therapist to get help.
I wouldn't worry too much, or blame yourself. Kids progress in different things at different paces. All you can do is the best you can, and it will be good enough. I know that I did everything "right" as far as promoting good speech habits (story time daily, no baby talk, lots of talking to her throughout the day, etc) and my dd still ended up with a delay in her speech. Don't stress it, and don't push too hard to try to "fix" it. My DD is currently on a break from her therapy (we took the summer off, because she turned 3 in the spring, and therapy through the school doesn't start until fall) and it seems like her speech has improved faster after we took the pressure off.
If you have a smart phone or tablet, you can try download a talking animal app. My DD LOVES Pierre the Talking Parrot! She will sit and talk to him, and he repeats what she says back in a silly voice. It is great just to get her talking. ABC Mouse also has a free app with music videos for every letter of the alphabet. My DD and I sing the songs together, and it has helped her speech a LOT! Especially because you can pick a sound that needs work, and sing a song focusing on the sound.