my mommoms cat has bad weight loss does anyone no wut it could be

HELLO I'm soo glad I found this site. I needed some advice for my mommoms cat kiki. He is pretty old and is fixed and up to date on shots. My sister lives with my mommom and I stopped by yesterday to see them and I picked up kiki because he ran up to me which is weird bec u have never picked him up in his life bec he just likes my mommom n my sister but he let me which was odd. Then I started feeling bones in his back n his ribs its like he is starving but my sister said mommom spoils him with food every time he meows. So when I discovered how skinny he was I started questioning my sister on how he ate and she said he doesn't finish a wet food small can ever most of the time it gets thrown away. I'm very worried I'm taking my kitten to get shots in two days n I called n asked if I could bring kiki too n they said yes so we will see a vet but I wanted to no if anyone has ever heard of a siamses older cat with bad weight lost so much that it looks like he has been starving n one more thing we have lots of fields around us n kiki is a inside/outside cat. And his gums look perfect n regular bowl movement n no worms in site n has been flea n Wormed lately as well. Thank u for ur time