Weight loss of this kind can be a very serious issue. I would recommend
getting bloodwork done immediately, and then possibly seeking out a GI
specialist if you have one in your area. You may also want to try
Nutri-cal or a/d prescription food to help him put on some weight.

I lost an 18 month old cat to Hepatitis & IBD. I had ignored the first
signs of weight loss for 2-3 months thinking I was overreacting.

Good luck,
Kristin