I'm having a hard time getting my infant to stay asleep alone in his crib.

I will rock him to sleep and I know it takes about 20 mins for babies to comopletely fall asleep and then I'll try to gently put him down he will stay asleep for maybe 10/20 mins then he wakes up and fusses. My fiance keeps telling me not to pick him up every time he cries but hes only 2 weeks old i cant help it, and babies are used to being with mommy at all times? Hello 9 months in our bellies 😁