Confused about blankets and baby sleeping bags

Ni

My baby is nearly 4 weeks old. I've always using a knitted blanket in his moses basket. But I've brought him a baby sleeping bag but I've read different things about how blankets aren't safe and the sleeping bags aren't either. The one I've got is 0-6months 2.5 tog. He's still pretty small for his age will he be safe in it? Do they reduce the risk of sids? Or can I still use his blanket?