If you think he's pulled a tendon, you would be doing the right thing by hosing it with cool water and wrapping with quilted pads and standing bandages (not polos) to keep the swelling down, and keeping him confined. For the first few days, lots of hosing will help the odds of recovery. If he will tolerate it, icing it (for I think 20 minutes is what is recommended) every few hours is helpful.
Tendon injuries are tricky - bad ones can have very little swelling, and vice versa. Generally it's a good idea to get a vet to look at it.