First off... Is the cord tied in a knot anywhere from his belly to the sac? If not then leave him be... he should absorb some more yolk.
If its in a knot you could try to untangle it. But if that fails you'll have to cut the cord. I had to do this with one of my babies this year. She was real skinny and didn't absorb any yolk. So I had my wife tie the cord off with thread and then gave her a few days to rest. Maybe 2. Then I took an egg out of the fridge and let it get room temperature. I then separated the yellow yolk from the egg, mixed it up and used a children's medicine dispenser and pumped some yolk into the snakes belly via its mouth. Just like assist feeding.
I had to do this 3 times. Fast forward a month and she is still a difficult eater, but has survived and is growing at a great rate. I soon hope to see her change her feeding habits around but the yolk really did help her...