Testing the waters and pushing the boundaries. He is probably into the routine of how you put him to bed and into the routine of how your Mum puts him to bed. She has probably done it the same way since he started to have sleep overs and he just goes with the flow
Yep, Pie was exactly the same. She used to have three different sleep routines depending on whether she was at FDC (bed, with bottle & dummy, on rare occasions a pat) or at LDC (mat, no bottle, no dummy, no tears!) or home - bottle, dummy, blanket & favourite toys, me or DH lying down with her & waiting for her to go to sleep

These days she still varies, depending on who it is & where she is. She's better for me than DH because I am a bit firmer about when playtime is over. She's only in LDC now & still goes down with no bottle or dummy, but we aren't game enough to try it here at home.