He has a large tank of water and has been drinking fairly well.

Fairly well?How many gallons does he drink in a 24-hour period?

We are having the first of very hot weather this week as well, which
means he is spending a lot time standing in the stall.

Does he have water in his stall?

I weigh the hay and it's 20 pounds dry, I soak it and give it in
three meals a day. I've been throwing the majority of it away

But how many pounds do you throw away? What we want to know is how
many pounds he is actually eating.

as it sours in his tub.

So maybe you've answered your own question about how to get him to
eat? If you must use the tub be sure to clean it out thoroughly,and
try to drain more water from the hay before putting it into the tub.

Does he eat all his 2 pounds of beet pulp? If so,can you increase
the amount ?

I have hay on to soak tonight and in the morning I will try
spreading it around his paddock.

Do you let it soak all night?If so,I'm wondering if the heat you are
having continues into the night? Maybe you're producing hay beer.Is
there a way you can soak the hay for an hour while you're doing
chores,and then drain and feed?
Sounds good about spreading it around.

I wonder about trying apple cider vinegar as a flavor tempter....

Possibly.But it will be interesting to see if he finds his hay more
tasty without the sour smell.

I'd be even more interested in how many gallons of water he's
drinking.

Lorna