I know what you mean if they are a plodder, sometimes its hard to get them motivated, I have one just like that.
Just recently I have started him on Synflex supplement for arthritis, he has not been diagnosed with it, but in his previous life before he came to me, he had no exercise for his previous 6 years, now he is 8 and was getting slower, I decided to see if it was because he was uncomfortable in his joints.
Now two months on he seems to have a new lease of life, even doing bum tucks which were very rare for him, also he is more enthusiastic on walks and running about so maybe your boy is feeling the aches of old age coming on.
Just to add, my boy had know interest in swimming previously, and last week he surprised me by doing a full swim in the stinky pond on our walk, to be honest I thought it was someone elses dogs swimming towards me as I had never seen him do it before without being pressurised [that was at a Hydro Therapy pool and he hated it] I could not believe my eyes he looked so confident.
So maybe any water you have would get his interest, and it is great for getting them fit without all the running about.
Just a thought, good luck.