Has he ever been around a puppy? Are you willing to separate the dogs and rotate which is out, if things go south?
A warning about dobes, because I have one – they are NOT gentle with littles. You’ll have to work very hard on teaching a dobe puppy to be gentle with smaller animals – our girl is nearly three and still occasionally stomps my cat. They’re also tough puppies and are quite literally a full time job until they’re around 2 years of age…I’ll happy talk more about the breed if you want, but I don’t want to flood this particular answer with my babble. 🙂
added note he went swimming in the creek yesterday and now his foam looks like algae from the creek in it…..could he have creek water in his lungs? When he lays still no coughing…..when he is active….coughing and retching.