I usually tie a knot at about 1/2 inch from the belly and cut the sac if it hasn't shrunk completely and is still attached within 48 hours.
Things like that happen it's not the worse that can happen, since he is active I say he has very good chances just need a little more extra attention.
Main thing you need to worry about is during those next 48 hours in the deli cup is that the yolk sac does not dry off (Hence the use water in the deli cup)