Know What Motivates Your Dog
Does your dog respond best to food, toys, or attention in general? You need to know what "currency" works best for your dog to modify his behaviors. Once you know what your dog likes best, you can use that as a tool to being your training. Get some different treats and toys to test out until you find the one your dog loves best.
Start with Simple Commands
Because senior dogs take longer to train than young dogs, you may have to start with small tricks and work your way up. Basics like sit, stay, and lie down are great starting points, no matter what age your dog may be. If your dog has a hard time paying attention, you first need to work on getting him to look at you during a command. Put the motivation tool near your mouth when you speak, and then reward him when he looks at you.
Work in Short, Repeated Sessions
It is easy for any dog to feel overwhelmed when you try to teach him multiple tricks at once. For senior dogs though, it can be even worse. Work in short sessions where all you focus on is one trick. Repeat this several days in a row until your dog finally gets the hang of it. Continue repeating the training for at least a week until you move on to something else.
Build on What He Knows
If your dog is already familiar with certain tricks, see if you can build on what he currently knows. For instance, you could turn the "lie down" command into "roll over" by simply teaching your dog to move in a rolling pattern on the ground. Building on these tricks, layer after layer, will eventually turn your senior dog into a true pro of commands. Good luck!