<human>: Can you give me a shortened version of the text?

So my roommates son just turned three two months ago, and I have noticed that he is well behind others his age. He can't say more than ten words, is clueless when it comes to colors, and doesn't seem to even understand what we mean when we ask him questions (example: I asked him if a shirt was yellow, and he responded "yeah yellow." I then asked if the same shirt was blue, and he said "yeah blue.") 
It is incredibly frustrating for both of us, and it isn't fit lack of trying, as we both spend several hours a day attempting to teach him, using a variety of tactics, from flash cards to games. Apparently his doctor said he doesn't have any indication of any mental disabilities, which leaves us all the more baffled. Any ideas on how to get through to him? He is a sweet kid and it makes me sad to know how behind he is.
<bot>: three year old is very far behind, doesn't appear to have any handicaps, yet we can't get any results no matter how much we try.