Development milestones

My premie was born at 33.6 weeks and he is now 4 months old. I know premies are behind in development but I was wondering how long it took for them to complete milestones or catch up for others? Right now he can hold his head but it is still wobbly. When I’m tummy time he is still unable to lift his head or push up off his arms and he hasn’t found his feet yet.

Positives he is almost 18lbs 😳 and he can mimic fart noises and puts his hands in his mouth.