I'm really glad to hear that your boy came through the surgery, and glad you had the surgery done!
Now at least, he will have less pain and be more comfy no matter how much time he has left.
There are things that you can try to help him feel better and that may keep the cancer from spreading too fast. When I had dogs with cancer, This is what I put them on: (I'm not saying anyone else should do the same as I did. I'm just sharing what I did, you need to do your own research and be sure that you look up all warnings about anything that you find!)
IMO this was the safest way to go. Did I buy them more time? Well my vets and I think it bought my Boxer 7 months he wouldn't have had normally. He had a huge tumor in and around his heart and had to have the fluid drained from the Pericardial sack around his heart, 1 time every other month.
Vit. B complex, 50 mgs. twice a day (with food) Vit. C. 200mgs a day.
1 cup of Essiac tea. (mixed in with food.) 1 tablespoon of Echinacea 2 times a day., Milk Thistle 500mgs per day. Vit. E 200 to 400 iu, Fish oil 1,000 to 2,000 mgs.
I also put them on a totally raw diet: Buffalo meat, heart, liver, one raw egg, goat yogurt and I juiced their veggies: Carrots and spinach.
Here is a great article on grain free dog foods: http://www.dogcancer.net/nograin.pdf
And lots of petting and doggy messages! They love that and it will help relax both of you.
Good luck and lots of well wishes.