What about lying down with him? Telling him it's time for sleep, mummy's going to sleep see? That worked for our DD for a while...

At around 20mths or so, we started setting some ground rules with DD... One of them was that when me or Dh puts DD into bed and says "time for sleeping", she has to stay in bed and stay quiet. I don't tell her she has to go to sleep as such, but the rule is stay in bed and stay quiet.

Your DS might be a bit young though...?