Hi everyone, My newborn will be 4 weeks old to...

Hi everyone, My newborn will be 4 weeks old tomorrow. He has been doing this cluster feeding for about 3/4 hrs some days where he feeds on and off each boob in this time. It goes from about 4 till 8 pm I also top him up with expressed milk or some formula milk (as we r mix feeding for various reasons) Anyone got any tips on dealing with this? It seems like he is never full :( has anyone tried nipple shields? Will this help with flow? Many thanks and apologies if I have all ready asked this before. Also, how long does the cluster feeding last?