Extra-tall penguin bollards all along the front of the sofa (you'll probably need about 36 for an average 3-seater, to make sure there's no room for his legs between any two of them).
Keep the 36 padlock keys on your person at ALL times and, whatever you do, don't EVER let him get his hands on them. You may wish to get another few hundred identical-looking fake keys to keep with them on the bunch to thwart him, just in case he manages to get them from you when you aren't looking.
Every time you want to sit down, simply choose which seat you want to sit in (rotating the seats you use will prolong the sofa's life), find the correct 12 keys, unlock the bollards and sit down. Use 24 or 36 keys if you have guests, as appropriate.
Make sure you lock them straight back up again whenever you go to the toilet or to get a drink if he's in the house with you.
Don't be harsh, though - get him an upturned orange box so he has somewhere to sit as well.
I completely understand your pain - we got a bed that we wanted to display in our DS's room and, every single night, as I live and breathe, he insists on SLEEPING in it.
Males are despicable creatures.