I suggest that this is one of those things that you just cannot control and that if you stop paying attention to it he'll eventually stop doing it. At 2.5 he's learning about his world and will try out many different things. He most likely started out chewing his food for longer lengths of time because he liked the way it felt or tasted or something else that interested him. Now, his chewing has become a source of attention from you. Now he's trying to figure something else out. He may be wondering, on an unconscious level, why you're so interested and focused on his chewing. He is hard wired to question and explore. Let him do it.
It's fair to say no desert until you've finished dinner, as long as you've given him a reasonable amount to eat. My daughter handles the not finishing dinner part by saying, you can have some ice cream after you've had 3 more bites. This gives the child some control. Then pay no more attention to him and his chewing. When he asks for the ice cream, say, oh,oh, you haven't finished dinner or perhaps, you can have it after you eat 3 more bites but do not mention that he has food in his mouth.
If ice cream is not on the menu don't use it as a bribe. Bribes are another way of being in control. You want to share control when it's appropriate with your child.
You do not want to get into power struggles over food. Doing so has the potential of creating long term negative issues around food. Ignore the chewing. It really is no big deal.