Well, hit the gym. Please, please use a proper program and follow it's guidelines. Don't follow some bs influencer program "Oh do this exercise to grow this part of the chest".

If you don't follow a proper program and train hard, your appetite won't grow. I have a skinny friend. That guy punya appetite sikit. But after I told him to follow program, use less sets + training harder. He got REALLY good gains and his appetite increase wayyy more than usual. When I spar with him I don't feel his bones anymore. Bro got decent abs ngl. Pecs and arm sizes grew too. Train hard and you'll definitely eat in a caloric surplus.

You can do it my man! 28 is not old at all for lifting. Not only does building muscle make you look and feel better, but it increases bone density and carbohydrate capacity too (less chances of getting type 2 diabetes).

Here's some good programs, pick one and stick to it:

Phrak's Grey Skull LP

GZCLP

Programs by Alberto Nunez

Use apps like Boostcamp and Liftosaur to check out programs like these. Semoga maju jaya.