You need to consider palatability and digestibility. Your dog is sure to lick up some of the toothpaste when brushing him so ensure that the toothpaste you buy contains no harmful chemicals that are detrimental to your dog's health when ingested.
You should consider the size of the toothpaste. It is more advisable to go for sample products or products with smaller sizes so that you can try out the product and if it works well for your pet you can buy the bigger size the next time you go shopping for pet supplies.
Other things to consider include ease of use, safety for your pet, and the packaging (does it come with pads and sponges or does it come in liquid gel or powder form?)
Toothpaste that come in chicken or beef flavors would be more suitable for your dog as the taste and aroma could be enticing to your dog. This way, you would struggle less when giving him a mouth wash.
The ideal toothpaste for your dog should be desirable by your dog and easy to use.