No Senna-col is not safe to use in dogs nor are most human laxatives.
This site has five pages of info on constipation in dogs
What I'd suggest instead of a human laxative would be feeding the dog a high fiber food if the dog is still eating for you. A spoon or two of oatmeal, plain canned pumpkin, or bran flakes (no raisins!) should help with moving simply constipation along.
If you have some on hand you could give some cat hairball remedy such as Laxatone or Petromalt. A small dog would be dosed same as a cat.
You may want to try a slightly longer walk than usual if the dog is healthy enough to do that as walking can help the intestinal tract move stool along.
But a dog can be constipated for many reasons including swallowing a non food item so if home care doesn't help I'd plan on seeing a vet.
If your dog might have swallowed a non food item, appears dehydrated (see if a pinch of skin over the shoulder blade stays stuck in a tent form or flows back down normally when released), has gums or tongue that look white, blue, gray or yellow tinted where they are normally pink, has a slow color refill if you press on a pink area of gum or tongue, or is becoming lethargic and non responsive to you then you may want to get the dog in to see an emergency vet if one is available in your area.
This site may help you locate an emergency vet if you need one
Hope this helps you!
Colloxil I don't recognize
and metamucil can be used
scroll down on the pages for information on cautions in use and dosing.
another mild laxative for dogs is cows milk which has the added plus of being fluid which can help if he's constipated from being a bit dehydrated. A half cup might work.