milk is full of sugar and it rots their teeth as they sleep.....I would just give him water or preferably nothing. if it starts that way at your house they think that's the norn....I have a 17 month old that HAS to sleep with two soothers at home.... hes never had a soother here ever. mom sent them on the first day and I put them in his drawer and never touched them since. I too wouldn't want milk in the play pen ...cause now you have to clean the play pen and likely it will stink anyway as it gets in the mattress