First, has he been to the vet since he got back? I am wondering if he might have been injured while he was lost, and is in pain, which is why his behavior is different. Also, I'd not keep up with the dominance training, as you describe it, and go with a more positive approach. First get him a vet check, and be sure there's nothing physically wrong, and then start over with his training as if he were a new dog, redoing leash training, and basic obedience, you need to reestablish the bond of trust you used to have with him. Positive training, praise and toys for good behavior, no toys, no treats no attention for bad, but this behavior change is so sudden, it makes me think he might have been hurt.

Also, of course, you have had another big change, there's a new baby around, so everything's different in that regard, too. What's your pup's name? Just curious!