How do I fix sagging sofa back and seat cushions on a leather couch?
If this is only a year old I would think it would still be under warranty from IKEA.  I would call them and ask what you should do.

Normally the manufacturer will send you some foam (or whatever) inserts.  Then you would add them to the cushions.  If there is no easy way to insert the inserts then the manufacturer might have someone with expertise visit your house to help.

If you want to fix this yourself and want nothing to do with IKEA then figure out what is in your couch and get more of it.  Then cut open the couch where need be and staple/sew it back together.  You are dealing with leather so this is not easy at all.