At 2 1/2 he really doesn't understand what it is he is saying. He understands how he feels and trying to put those feelings into words can confuse everyone in the area. Chances are he is upset that she left in the first place and doesn't know exactly how to express that combination of feelings. He understands many words, but putting said words into sentences doesn't always come out right. So feelings that say he is upset mommy left come out as he doesn't want mommy to come home yet....etc.
"Our best successes often come after our greatest disappointments" -Henry Ward Beecher