This has been going on for a while now. At first, I thought it was just a coincidence that he uses past tense, but then it became apparent that he is aware of the different words and its usage.
For e.g., he will say "I drank already", "Yes, I did that" and it is really impressive for me as a mother. Of course, he does not always know all the past tense words and so, sometimes he will say things like "sleep-ed", "think-ed" ... basically he just use the present tense and adds in "-ed" to make it past. Now, isn't that impressive? And he is only 4 years old!
I am really pleased with him right now. I just think the world of him ... and I cannot stop smiling whenever I think about it too!!