and welcome to TCS!
Cats are naturally limber, active, playful creatures. Being overweight effects them physically for several health reasons as well as inhibits activity and mental effects like depression, OCD, anxiety......
My first thought is have you been consulting a veterinarian? How long have you had him, how old is he? It also depends on his breed and build as to what his weight should be. Have you taken him to a vet since you adopted him? It is best to work with a veterinarian when it comes to such a serious weight problem. It could even be caused by another health problem.
30 pounds is excessive for a house cat, and I praise you for your concern.
Also what are you feeding him, how much each feeding?