If your son is 8yo and has encopresis, is there a type of diaper I can buy in order to prevent the smell?

It's tough. If your are trying to get your kid to stop being encopretic, sometimes diapers, in my experience, kinda coax kids to just soil more, as they realize they are wearing diapers. Encopresis can be very challenging to manage. I have various strategies for the kids i see, and my best advice is to have you and your son work together with a specialist. I think the smell is just going to be part of it.
Sure... ...But it would be better to actually treat the encopresis. Cleaning out the bowel with several enemas in a row followed by taking mineral oil or MiraLAX (polyethylene glycol) by mouth, and setting enforced toilet times in addition to as-needed toilet, should work wonders.