It happens. When the dog is doing well with potty training the owners get complacent (even without realizing) and that's when we see accidents start up. Approach this as if you have an 8 week old puppy and keep up that intense potty training strategy for a solid 2 months before relaxing at all. That means constant, direct supervision any time the dog is out of his crate. The instant the dog starts to have an accident it must be caught and interrupted (not in a scary, intimidating way, just in an urgent "woops!" kind of way). So interrupt and quickly bring dog to his potty spot to finish. Every single outdoor potty should be rewarded and praised
right there in the potty spot immediately after he finishes (not once he's back in the house). Proper potties should be followed by an enjoyable activity such as outdoor play time (we don't want to send the message that potty = end of all fun, back inside IF the dog enjoys outdoor time. If on the other hand the dog hates being outside, then outdoor potty
should = back in the house.)
It's awesome to hear he plays well with your other dog. Notice any time he pauses in their play, wanders off, starts sniffing the ground, etc. - taking a break from play is a good indication he's feeling the need to "go". I'm sure you already know to take him out frequently, especially after he wakes up from a nap, after he eats or drinks, after a play session, etc.
You are cleaning accidents with a specially formulated enzymatic cleaner right? Douse the area, really soak it. Use that Nature's Miracle / Simple Solution liberally. Scent is powerful when it comes to where a dog potties. The scent of a previous potty triggers their brain to eliminate there again.
I have a good feeling about you for this dog. I seriously think you can do this, potty train the impossible-to-potty-train dog! Just think, putting in a couple months of annoying hard work will result in a lifetime of a perfectly potty trained pet. It's worth it. Treat the potty training plan as if you're dealing with a baby pup, don't get comfortable until he hasn't had a single accident for 2 months. When you reach that milestone you can start to gradually relax the plan a little bit. I think you know what you're doing, you just have to do it longer than would normally be required for an average dog.
The worst thing that can happen in this process is for accidents to go unnoticed. As you know, the act of relieving oneself is in itself rewarding. So if he is permitted to have that rewarding experience without interruption, it will form a strongly reinforced habit. We need every single indoor potty to be interrupted (and dog rushed to finish outside) and we need every single outdoor potty to result in praise and treats.
Again make sure when you interrupt his accidents it is just that, an interruption. Not a correction. It's "Woops!! Let's hurry and get you to the right spot for that!",
not "bad dog, no no".
I'm also hopeful that your other dog will be helpful in the potty training process. He can't stop the pup from having accidents but he sure can model the correct potty behavior (holding it inside, pottying outside.)
I believe you can and will be successful! Good luck and stay vigilant.