Cat has lost 3.5 lbs in a year

A year ago at his vet appointment, my cat weighed 16 lbs (he's a big boy). I recently realized that he looked skinnier, so I weighed myself and then weighed myself again with me holding him. He's down to 12.5 lbs.

I've made a vet appointment for this week, but I'm just wondering what if anything this could be. Some background info:

--Not sure how old he is (he was a stray). Probably between 8-11 years old.

--Eating and drinking normally. Fur looks healthy. Normal behavior.

--He does occasionally go outside. Maybe he got into something or has a parasite?

I just lost another cat last year to cancer so I'm really hoping it's not that. 😭