This is how one Hometalker fixed theirs
You need to fix the bottom if the cushion. You may need to add some more strapping under the cushions.
(Foam for the purpose can be purchased or fitted for you by an upholsterer)
If the cushions are loose, you could try adding a seat pad into the cushion cover and or add one under the cushion too. Failing that you will have to replace them with new cushion fillers.
You can try opening up the cushions (they should have zippers) and adding some additional stuffing. But honestly this is probably a short-term fix.