Easter is a special occasion, it overrules regular and holiday time, but I wouldn't make the argument that the Easter period shouldn't be included when tallying up the number of nights in the holidays to work out where the halfway point falls. I would simply accept that "halfway" doesn't necessarily mean "an equal number of nights", so dad might get one or two extra nights here and there for those special occasions. You'll find the same for Christmas in the summer holidays.
So what would I do? I would collect the child at 3:30pm on the middle Sunday of the holidays, then drop him back again at 5pm on Easter Sunday, then I would collect him at 5:00pm on Easter Monday. Yes, dad will end up with 10 nights instead of 9, but it's Easter, it's one night, it's in the orders, and the child doesn't see him as often.
For the record, when things were bad between my husband and his ex, even their very rigid and watertight orders were being interpreted to either benefit her or cause conflict. Don't get tied up over one night. The conflict is going to do more damage to the child than one or two extra nights with his father.