An older dog is likely to be resistant when a foreign object like a toothbrush is put in his mouth. If any kind of pressure or aggression is shown the whole situation will become intolerable, and despite good intentions any chance of a successful outcome will be lost.
Before introducing the toothbrush, allow your dog to smell and taste the toothpaste or gel. This should not be distasteful. If there is reluctance to do even this, put a little peanut butter or something you know he will enjoy on your finger and let him lick it off. Lift, pull back the upper lip and gently do the same while swiping your finger along the gum-line. Soon your dog will relax and you can do the same thing with toothpaste or gel. This may well take several days, sometimes more before a good teeth cleaning can be performed.
Once you feel your dog is comfortable letting you run your finger along his teeth and gums, it might be time to try the toothbrush. Put a little toothpaste on it and allow him to smell it. He will be familiar by this time with the smell and the taste. Praise him and start by gently cleaning one or two teeth. Stop frequently, continue praising and try again. This may also take several days or even weeks, but persistence is the key. Do this every day, preferably last thing at night, as the gel or toothpaste works well with his saliva and so drinking afterward is to be avoided if possible.
If you do this properly, it's possible the training will become an added quality time together. Your dog will look forward to this new habit and you will both be rewarded, him with clean teeth and you with a sense of accomplishment and a vet-free bill.