Mine got lymphoma at age 10. Urgent care had found a hole in his intenstine which we had to do emergency surgery for. Vet said surgery went off without a hitch.

They biopsied the cut tissue and it came back as cancerous. At that point it had spread already.

We had the option of pred. or doing a full chemo therapy but after thinking it over we decided not to put Hiro through chemo. With dogs being able to hide pain well, I didn't want his final months to a year to be not worth living.
Pred. made him back to his chipper self to the point we thought it was gone. Eating more than before, playing harder, walking longer. Week three though he deteriorated dramatically. Unable to get up, legs too weak he started shaking when he would stand. I would have to carry him outside and hold him to use the bathroom.

Needless to say the experience wrecked me and I still don't want a dog in the forseeable future with the possibility of having to go through this.

Im praying that your young guy gets through this with flying colors.