He is as active and happy as he was before but he has been sometimes only eating one meal portion a day. We thought it was that he didn't like the new food for weight loss but he hasn't been eating much- 2 meals a day. He is a 8 kilo (measured at home) 4 years old Norwegian Forest Cat. He was always a nibbler. He also hasn't been going to toilet as much as he used to a previous drought of 3 days without defecating as far as I know. Not often peeing either. What sort of conditions should I concerned about? He is also not drinking as much.
You could determine whether he's showing his unhappiness with the new food by checking whether he eats and drinks better on his old food.
This is shedding season where I live. If I don't comb my cat thoroughly once a week at this time of year, he gets hairballs. Your cat might be constipated. You can buy some hairball medicine over the counter at the grocery store. Dab it on his paws so he licks it off. This acts as a gentle laxative.
Your cat might have a parasite. You could collect a stool sample and take it in to the vet's office for them to test. You would then be able to treat for parasites at home.
When I am unsure whether to take my cat to the vet, I call and talk with the technician (veterinary assistant) on the phone. I describe my observations, they ask a couple of questions, and then they triage us -- advise whether/when to come in, what to be watching for. I hope your cat has a regular veterinary office you can work with in this way.