Dog's toenail is swollen and hard. When should I take him to the vet?

Ouch. That does look swollen. Dogs can get bacterial or yeast infections around their nail beds. If that is what is going on, the Epson Salt soaks will likely slowly make the swelling go down over a couple of days. However, if Tom is licking the nail, it will likely stay moist and have a harder time resolving. Unfortunately, there are other conditions that can occur around nail beds like tumors and auto-immune problems, so if it is not getting better, is getting worse or seems painful I would take him to your vet. i hope that answers your question. Thanks for using Petco Pet Education Center, formerly Petcoach.