I have a 12 year old male, On sunday I rescued a 6 week old female kitten. I stayed home Mon & Tues.?

At first when she came in, he was not happy, she is also very tiny. But on Tuesday, he seemed to enjoy watching her play. And when she came up to him he didn't growl anymore and didn't hiss. He actually looked like he wanted to play. He tapped her on the head a couple of times but was gentle about it. When I rescued him he was 8 weeks old, and at the time I had 2 sisters, but unfortunately I lost both of them due to age, so he has been without another cat for 3 years now. Today I had to go back to work, and when I came home he wouldn't eat his dinner and looks depressed. I actually got the kitten for him hoping that he would have a playmate during the day while I was at work. He threw up twice and is having some Diarrhea . But he was eating some of her kitten food. Would that do that?? Any advise??