Thank you for the follow up.
I agree that some of his changes may be age related and may have been more apparent after the kitten was added. I have seen a good number of dogs affected the condition known as cognitive dysfunction and all have exhibited different signs, especially in the early stages. I am glad that your veterinarian did not identify any physical problems. If that is what is happening, there are some other things you can do that may help.
Some dogs I have seen have responded to diet change alone. Hill's b/d diet is a good choice. LINK HERE If not, the medication called Anipryl® is very effective for many dogs.
I generally start with supplementation first (+/- diet change to b/d if there are no other dietary restrictions needed). I usually recommend increasing the omega 3's in the diet as well. The active ingredient of fish oil is EPA (Eicosapentaenoic Acid) and DHA (Docosahexaenoic Acid). You just want to make sure you are seeing those on the label as an ingredient and not just the words "fish oil" as these are the important part of the fish oil and not all fish oil capsules have them in it, especially the cheaper ones. Aim for 180mg EPA and 120 mg DHA per 10 lbs body weight daily. There is a large range of safety and it doesn't have to be that amount exactly but this gives you some guidelines.
Another product that may be of value is the product called D.A.P. It is a synthetic pheromone that works to create a feeling of calm in a dog's environment. It is available in different forms. LINK HERE
There are various additional supplements on the market to help with brain health. The three most readily available are: Cholidin, Senilife and Novifit. As far as which supplement to try first, I do not think there is any general agreement on that. Between the cholidin, senilife, and novifit I would make my decisions on which to use based on cost, ease of administration and availability. I do not see a problem with using them all if cost is not an issue and you have a dog who doesn't mind taking oral supplements. There may be more benefit to starting with multiple approaches for a greater combined effect and backing off to a minimum effective combination if you get a favorable response. It is not always easy to track response as the symptoms can be variable and intermittent.
I would add the Anipryl if needed for better effect or as the cognitive dysfunction progresses.
Here is a link with more information about cognitive dysfunction in dogs. LINK HERE