I think it helps to go back to first principles. What Kohn is against is evaluating a child or what they do (positively or negatively) because when someone, especially someone as important as a parent, does that it teaches the child to override and disregard their own evaluation. So I think reinforcing DS's own self-evaluation which he's already come to is OK ("I did it! Yay!" "You did! Fantastic!" - or Floridama putting her DD's report card which DD is proud of on the fridge) and I think telling him how I feel about him ("I love you and I think you're the most wonderful little boy in the whole wide world and I love being your Mummy") is fine. Neither of those seems likely to stop him learning to evaluate what he does or to trust his own evaluation. I also try to be interested in what he has to say and to show me and to take a positive approach to it, of course.
What I try to avoid is jumping in and praising things he hasn't noticed, or phrasing things as evaluations of him which are actually expressions of my own feelings. (E.g., I try to say "Thank you for emptying the dishwasher, that lets me get on with supper" rather than "That was so responsible of you, emptying the dishwasher" even though the latter is the kind of "specific praise" that some authorities approve of.)
