Worried about my cat

My cat Mercutio is about two years old (he's a rescue, so we aren't sure exactly how old). He's always been pretty active, but over the past week I’ve noticed some changes that are worrying me. Lately, he has been eating less and seems to lose interest halfway through his food. He still drinks water, but not as much as before.

He also seems more tired. He spends most of the day curled up in one spot instead of playing. He tracks the laser pointer with his eyes but doesn't seem to want to chase it.

He still uses the litter box, but his stool is smaller and less frequent. I haven’t seen vomiting, or obvious pain, but he sometimes sits hunched up like he’s uncomfortable.

He’s an indoor cat, so I don’t think it’s something from outside, unless the dog can transfer something. We use Frontline.

I know no one can diagnose and it could be a whole lot of things, but what are the possibilities? Or what signs should be the point where I stop waiting to take him in? Am I already there?

I do have an appointment scheduled (Monday), I'm just trying to decide if I need to bring him in sooner. It would cost a lot more for an emergency visit, though, and as much as I hate it, I have to consider that. Things are incredibly tight, or I'd be there already.

I'm so worried about my guy. I'm just not sure that it can wait until Monday. I guess I'm just looking for someone to tell me that bringing him in sooner the right thing to do, even considering the potential cost of the visit and whatever else might be wrong.