Some things I have noticed recently:
He is using a lot more phrases like, "We did it!" and "Let's do it together" and "Can you help me with this Mum?". It shows he is seeing himself as a partner in our activities, and he is therefore participating more. The other day he stayed for a full half hour helping dad and sister wash the car.
Personal likes and dislikes
Yesterday he spontaneously came out with "I like ham sticks Mum" when I gave him his lunch. Expressing likes is not something he has ever done before.
Shame on me - I lost it yesterday and really yelled at the boys. (They were killing each other). Instead of completely melting down he looked with big eyes and said, "Mummy cross!"