My suggestion is to download Equilab Omaha from here and learn how to use it.
Then you should think what kind of hands opponent might have there. You don't have to know the exact hand, that would usually be impossible. Instead we talk about ranges. When BTN re-raises there he propably has a flush. CO has either a flush or a set usually.
So if we give them those ranges, it would look like this:
CO: *s*s**, 66**, 22** (any hand with two spades, any hand with either pair of 6's or deuces)
BTN: *s*s** (any hand with two spades)
When we insert those ranges to Equilab, it shows that hero has 33% equity on the flop. It looks like hero can call, but to be sure we need to calculate it. To make it short, hero should call.
To make it longer answer, here is the math:
Spoiler
There will be main pot and side pot with BTN.
Main pot is 2.27 and hero has that 33% for that. Hero needs to invest 0.11 more to it and his EV share is 0.75 so it's +0.64.
For the side pot with BTN hero has 35% equity. Hero invests 1.21 to 2.42 pot and his EV share is 0.85. That would be -0.36.
When we add those the total EV would be +0.28 so hero should call.
edit: fixed my brainfart for main pot.