18 month old can't drink sitting up

Hi,
I was just wondering if anyone else has had an 18 month old that can't/won't sit up while drinking from their sippy cup?
He was initially delayed with rolling, sitting etc but then all of a sudden picked it all up and was walking by his 1st birthday. It's making it really hard to start the transition to a regular cup, he's not my first bub but none of the other kids did this!