No more peeing outside the box!

My cat has been peeing outside the box ever since we adopted him. We tried everything- getting 3 litterboxes for our 2 cats, trying different types of litter, different types of litter boxes, cat attract litter, Feliway diffuser, and I clean their boxes 2-3 times a day and deep clean their litter boxes frequently. We got him checked at the vet and it was determined it was behavioral. We've had to put pee pee pads and waterproof blankets all over the couch and constantly supervise him. It was a nightmare. He got prescribed gabapentin (100mg twice a day) and Fluoxetine (5mg a day) and he finally stopped peeing outside the box! It's been 5 days and not a single accident so far. I could cry. He's also been less aggressive with our other cat. The only thing that worries me is that he's been less cuddly now and seems a lot sleepier. I know this is what the vet recommends but I'm worried that it might be too much medication for a 9.3lb cat. My anxiety is taking over now haha but at the same time so happy that he stopped peeing outside the box!