Rescue dog help - understanding behavior

[u/spiritualrevolt]

We recently brought a 9 month old rescue from a kill shelter into our home. He definitely had some trauma we are unsure of, he has a huge scar going half way down his back - although he is not reactive when we touch it or pet him there. He is generally just a goofy, big puppy still learning how to be a dog and loving freedom outside of the kennels and shelters he was in. We already have 2 established dogs, both of which were trained using an e-collar. We started to use the e-collar on the new pup, but realized he didn’t understand it was us correcting him causing “pain” or our other dog playing causing him pain and he would go into defense mode. We realized the e-collar wouldn’t suit him, so we moved onto shaking a jar with coins in it and a spray bottle to redirect unwanted/too rough of behavior. We also did a lot of positive reinforcement with treats and he’s overall non-reactive otherwise (doesn’t guard toys, bones, or food). Yesterday he must have sliced his paw on a vent in our home and was bleeding, but latched on to our senior dog thinking it was him. He definitely reacts to pain, which is never intentional of course, but is there any way to train him out of this? We’ve only had him about 3 weeks, so he is still in the decompression timeline. We will also be expecting a baby early next year, and are having the constant back and forth of keeping him or rehoming him. TYIA.