Can I decline this, and if so, how?
While the other answers cover how you should probably just accept, I'll take a stab at actually answering this part of it. You definitely can decline a gift and do so professionally. The key points to focus on are gratitude and not making a big deal of it.
That's extremely generous of you but I would honestly feel bad [/ a bit awkward] to accept such a large gift.
In your situation and given that the tickets are already purchased I'd recommend meeting somewhere in the middle and adding:
I'd be very grateful to use those tickets but if you don't mind I'd prefer paying my own way once me and [person you're going with] are there.
You can bring up how you're not used to accepting such valuable gifts or how you've been brought up to pay your own way. Making it about a personal quirk of yours is always a good strategy to adopt, even in a case like this where you shouldn't really have to explain.
It's a bit weird but I've always taken pride in providing for myself and being independent. I'm blown away by this kind gesture but if you don't mind I'd prefer to pay my own way.
Of course, if your boss insist you'll just have to accept his generosity. If you feel strongly about this or get the sense that your boss is disappointed you can suggest that he donate the money to a charity of your or his choice instead.
You'll also want to consider whether this is something he does routinely for all his employees. If that's the case you should consider this an informal and variable part of your compensation package, a sort of in kind bonus. It doesn't make sense to refuse it if that's the case.