ok, I found information on the medication you've been giving him this past week.
Zylkene is a natural product derived from milk proteins that has been proven to help your cat or dog cope in times of stress and adapt to change. Can be used for short periods or continuously if needed.Zylkene is a revolutionary new product used to help cats and dogs cope with stress. It is a food supplement made from a protein found in milk (the same molecule that helps babies relax after a milk feed). The active ingredient is a peptide (a protein molecule) which is able to bind temporarily to certain receptors in the brain. This has a calming influence similar in some ways to tranquillizer drugs, but without the side effects associated such as sedation or memory loss. It has been clinically proven to be effective in dogs and cats.
It can be used for the following situations:
- Environmental changes - e.g. new pet, new baby or house move
- Multicat households
- Cattery or kennel stay
- Home Alone
I found it off this site: Zylkene
I know this sounds weird and you said you've gotten a cat flap where only your cats can enter (good!) BUT are THEY aware they are the only ones that can enter.
My guess is no. It tends to make a difference.
My cats all know they come in at night and I lock the cat flap. No one can get in and they can't get out. They know they can't gt out, so they associate that in their head with nothing being able to get in.
They feel secure at night and sleep/ lounge all over the house and no one has toilet issues.
Perhaps if at night you kept them in and maybe blocked the cat flap (peice of cardboard over it held by tape, or box in front of it, you get the idea) something they can associate with feeling secure.
It would also require setting up a routine which is another thing that helps in them feeling secure. (just like children) Let them out every morning, same time, have them come in for their last meal, block the cat flap then. (just a suggestion)
That combined with your medication may make a difference.