My dog barks at other dogs. How do I introduce a new puppy safely?

Poor Merlin! It sounds like he gets very upset around other dogs, which can be a challenging problem to work on in some cases. You will need to take things very slowly when introducing him to the new puppy - this will be hard for both of them, so be patient and stay positive. For now, I would recommend keeping them separated unless you are actively working on training with them - this will help to prevent any setbacks that may occur if they get into a fight.

I would try to do several short training sessions each day if you can, no more than 5-10 minutes each. You will need another person to help you, and both dogs should be on-leash. Start with them on opposite sides of the room, and give treats for calm behavior while they are in the same room together. Over the next few days, if things are going well, you can gradually start to allow them to be closer together, still giving treats for calm behavior - if either dog becomes upset and starts to bark or growl, just move them further apart again. It may take several days or even weeks before they can safely be loose together, depending on how severe Merlin's problem is.

If you are not seeing any improvement within the first week of starting this plan, I would strongly recommend getting in touch with a good reward-based trainer who can work with you in person. You can search for a qualified trainer in your area here: www.ccpdt.org