It sounds like he's making up for the long stretches between feeds during the day by waking more frequently overnight. Maybe try waking him for 3 hourly feeds during the day and he may just do the longer stretches at night.

We had feeding issues, DD wasn't gaining much weight and she was a sleepy newborn - also doing 5 hour stretches during the day. I had to wake her every 3 hours for day feeds (plus give her top ups of EBM but that's a whole other story ) and she quickly switched to sleeping longer at night. At around 5 weeks she'd put herself into a routine of asleep at 7pm, feed at midnight, feed at 4am then up at 7am. She quickly adjust to demanding more frequent feeds during the day too. It worked so well which was just a bonus really as we only did it to help with her weight gain and my supply issues, lol!

HTH

PS and a nappy change between breasts really helped us too as DD fell asleep frequently at the breast. I often had to strip her off for feeds too.