Wilson is a doll! I do hope you will keep sharing photos of him. We would love to see your other dog too, who must be so special to you after all these years.
The loose stools could be from the transition, but I have heard from many this same issue. Wellness is a great food, but not one of our recommended dog foods. You should probably finish the transition and wait a couple weeks and see if the stools firm up before deciding to change again too quickly.
Welcome to the site, glad you decided to introduce yourself and Wilson to us!