DS has this habit of behaving great until we comment on it, then going nuts. Today, for example. He was awesome this morning. He played with DD, picked up messes, made his own lunch, etc. Then DH came downstairs and I commented to him how great DS has been and told DS that if he kept it up, I'd let him use my lunch bag and take his lunch to school on Tuesday (it's a big deal for him). The second I said that, he went crazy. He started getting into things he wasn't supposed to, lying to us, arguing, whining, etc. The day's been crazy ever since.
DH and I have both noticed this. The second we say something nice or tell DS we're about to go somewhere nice because he's behaved well, he starts acting up. We've had to cancel outings more than once because DS wouldn't get dressed, refused to eat lunch, whatever. But if we don't comment on his behavior or mention an outing, he usually acts nice the whole day. Which means that we usually don't get to go places because the second he finds out we're going he starts acting up.
What is up with this? Isn't positive reinforcement supposed to be a GOOD thing? Why does DS start acting like a brat the second we say something nice to him?
Sounds like approach avoidance to me.
My dd does something similar - most days we have fun at home or doing our thing while her brother is at school. About an hour before he comes home she starts to get moody, cranky, etc. She adores her brother and is looking forward to him coming home but is also conflicted about his arrival, it is a difficult transition in our day. She also looses my sole attention. She often has tantrums about this time and generally makes life difficult.
So in your situation maybe it is your son becomes nervous about the reward or the possibility of loosing a reward that is SO important to him. Or maybe he is a strong introvert and although he enjoys outings they also make him quite anxious? It is unlikely he realizes or could put words to why he is doing it. I get that is is mind boggling frustrating.
What if the outings or special treats were mentioned as part of the plan for the day but with out an emphasis on a reward?
And maybe try giving him non-verbal positive reinforcement like a hug, high five, a smile, a wink, what ever he seems to respond to best?
When I was a kid, I didn't want to have to act a certain way to get my parents to love me or approve of me. I wanted to feel like they loved me the way I already was, not just the way they wanted me to be. When I was being obnoxious, doing stuff I knew they didn't like, I think sometimes it was because I didn't want to have to pretend to be a nice, obedient kid to get their love. I wanted to see if they would love me even when I wasn't acting nice. I wasn't consciously thinking exactly that at the time, but eventually, as a teenager or young adult, I was able to look back at my younger self and see what I had been trying to do, and why (of course) it had never worked out the way I wanted it to.
With that in mind, if I were you, the next time I made a nice comment and my DS started acting terrible, I'd give him a hug and say something else nice, and then I'd say, "I know I told you the special outing was going to be a reward for being good, but let's do it anyway even though you're not being so good anymore, because I love you just as much when you're bad as I do when you're good." (And if I tried that approach a few times and it didn't help, then I'd give up the idea that he was like me and I'd try something else.)
You should read "Punished by Rewards" by Alfie Kohn and check out the articles under "parenting" and "rewards and praise" on this website: http://alfiekohn.org/articles_subject.htm#null.