My cat appears to be losing weight and having accidents

My cat is a fixed male, almost two years old. Although he has always been a naturally thin cat, it appears that he has lost a little bit of weight. I have dry food and fresh water available for him all day and when I noticed him losing weight I started give him wet food at nighttime so that maybe he would gain a little weight. Earlier this morning I noticed some wet spots on my bed where he had been laying and I assumed I had accidently spilled some of my water. But a few hours later my mom told me that she had just caught him peeing in her bedroom floor. In the almost two years I have had him he has used that bathroom outside of his litter once, and that was the second day I had him and because I had accidently shut him in my room while I was sleeping. Other than that one time he has NEVER used the bathroom anywhere but his litter box, even when I take him to my sisters apartment or when we took him on a trip to the beach. We're very worried because this is not like him and it doesn't look like he's intentionally using the bathroom it looks more like he's having accidents  and I clean his litter box every day. I plan on making a vet appointment but its Saturday and I have to wait until Monday. What could be causing this?

He could have a bladder infection.  Keep a close eye on him and make sure he is still urinating through the weekend.  If he stops urinating take him to an animal ER immediately.  Otherwise he should be okay to wait until Monday to be checked out by the vet.  Something else is probably going on to cause the weight loss, you may want to have the vet run some bloodwork to check his organ functions.

I'll keep an eye on him! What is the usual treatment for a bladder infection? Is there anything I can do before Monday to help him? Thank you for your answer!

Normally depending on how bad it is it's usually antibiotics for 10 days to 2 weeks.  Sometimes pain medications.  Some vets have you change the diet, but usually not unless they have chronic issues.  My 2 bladder cats eat canned food (not a prescription) and I add a couple of tablespoons of water to their food.  You could do that add some water to your cats canned food that will help flush out the bladder a little bit.  this website has a lot of good information about bladder health.  http://catinfo.org/?link=urinarytracthealth

Thank you so much! I really appreciate it