What does he think of being with his "friend"? Maybe you could do a visit there and see if the environment is different there and also observe any changes in ds with knowing a child in class.
I agree with Zen Scanner...I think you're looking for a difficult-to-find balance of flexibility with an encouragement to be plugged in to what the class is doing and follow someone else's agenda (the teacher's).
It may be too much to expect more than parallel play for him right now...but I do think there is great value in gently challenging his 'zen' - since you know that he will be challenged increasingly as he gets older.
I think there's also merit in having someone else do the challenging...sometimes kids are more receptive to this than with their own parents.
