Awww, poor Merlot! If it's not bleeding, then it's probably not too deep, and that doesn't look too bad to me. If their pads are extremely dry, they can get them without stepping on something, but more likely he somehow stepped on something sharp (sharp rock?).
This has happened to Theo several times, and this is what I do: First, I clean it extremely well by irrigating it with saline solution, and then I clean it further with something like alcohol or Hibiclens. The safest thing is to keep it open so that it can drain if necessary. I would try to keep him from walking on dirty surfaces for a couple days, just to give it a chance to heal. For walks, I usually wrap it with vet wrap, and then tape it up with water-resistant athletic tape. You can also use a little bootie if you can find them.
If it's not deep and you KNOW it's not infected, you can glue it shut with liquid bandage or nu-skin or even Krazy glue (trackers do this in search-and-rescue situations when dogs get cut on debris), but that's a bit risky because if it is infected, it won't be able to drain. Many people on this forum swear by EMT gel, but I've never used it.
In my experience they take about a week to heal up.
