I have a girl that age whose parents just had a baby and she went through a week or two of tantrums and meltdowns. I stayed more firm than ever and gave her a couple of time-outs in the playpen which I had never had to do. Since then she has been very well behaved with me, but melts down the second her mom comes in the door to pick her up. It is actually very possible to have him behave for you even if he doesn't for his mom.