Blimey not got to live up to much then!
Sorry your dog is convalescing - a nightmare time for all of you!
It is very normal for some dogs not to play but does make it so much harder for you at a time like this. I would not give up on this though. Most dogs can be taught to play and this may be the time to do it.
Re treats I would give him weeny meals and make him earn all his food. It may mean having to use really yummy yummy treats hot dog, cheese, chicken, baked liver etc.
Without being too boring or predictable I would clicker train him. Clicker training for 10 minutes can tire out the most active dogs.
Once you get him accustomed to the clicker and for this do use yummy treats. If he is not that treat orientated just do it a few times when he is hungry. Seriously I would not give him his tea and then clicker train him before breakfast if necessary. So click and treat, click and treat etc. Obviously the clicker can be used for general training but if his movement is restricted do free play. Basically just give him an object. I do this with an empty box. Just put it on the floor and if he shows an interest in the box click and treat. He will keep looking at the boc or pushing it, he may stand in it but whenever he shows any interest click and treat. It is amazing what tricks the dogs show you they can do. Mine stand on a washing up bowl and walk around it on their front legs, one will pick up the bowl and throw it over his head one pushes it into a pretend goal. (oh what jolly fun our household is!!!)
Then change the object and again wait to see what behaviours he throws at you.
One of my dogs today in the snow kept lifting one of his paws to wipe the snow off his noise I clicked each time him did it and now bless him he will lift his paw across his face when I say "cry". Completely useless but he is proud of himself and it is dead cute!
Another way to amuse dogs is to take them for a ride in the car. (If you can get him in the car!)
This also may sound a stupid suggestion but some dogs will watch telly, generally not so much the retrievers but collies so worth a try one man and his dog is a good one for one of mine and the monkey world program is also one some of mine will watch.
To get him toy focussed I am afraid you have to behave a bit like a loony but if you get someone to hold the dog and you play with a toy in a mad cheerful and interested way with your back to the dog. Makes lots of high pitched squeals of enjoyment and do not on any account let the dog near you. Then let the dog off to come to you and send the dog away or move away from him with the toy but still being excited. The more you don't let him have the toy the more he will want it. Do then let him have the toy but he will probably not know what to do with it so you will have to pull it around or hide it under your feet etc. (Do warn the neighbours and OH as you don't want to get taken away by the men in white coats for your weird behaviour!) Then stop the game when he is interested and do not let him have the toy. Put it away and then later do the same thing but this time with a different toy.
The toys can be made out of socks stuffed with socks, old tea towels tied in knots but you may find a furry sqeaky toy may get a retriever going.
I'll think some more and get back to you.