needing some support or insight

Anais
Hello ladies! Im looking for some insight or support with a BF issue that's been troubling me. My son is 5w old and for the last two weeks in the evening he wants to eat more then normal. Usually he nurses for about 20-30 min but I'm talking hours... after a while he gets frustrated and starts tugging and biting my nipples. He even cries out a bit. The last week I've given up (so to speak) and given him expressed milk or formula (depending on what I have) and he will eat up to 6oz AFTER nursing!! This leads me to believe he is not getting enough at this particular feeding. I am someone who struggles with supply. With my first some I dried up at 4w... this time I'm determined not to do that! All the other feedings go (seemingly) well! Is this normal? Any advice on how to get though this evening feeding??