confusing words

My 27 month year old gets the words, "on & off" & "up & down" mixed up. I am concerned for a couple of reasons, he is very smart and articulate, and speaks well and has been mixing this words up for at least 6 months now even though he knows what they mean. When he wants me to pick him up, he will say, "up and down" instead of saying "up" like he doesnt know which word to use or when i am holding him in my arms he will say "up" and i say do you mean " down"? and he says "yes,down" These are coman examples of how he uses the words incorrectly. Also with the words "on & off" he does the same thing, he will say, "shirt off" instead of "shirt on". I guess i was wondering if this is normal or if i should have him evaluated, the only reason it concerns me is because it has been happening for about 6 months or so and becasue he is articulate and speaks well so this seems out of the flow or character to what he should be doing, well more like he shouldnt be doing this and that something is wrong. Any thoughts would be appreciated. Thank you!