Jack Be Nimble, Jack Be Learning Quick!

So here's Heart Attack Jack....
Heart Attack Jack? Yep, that was his racing name, but we're just calling him Jack - no heart attacks involved here! Jack was welcomed into the house for fostering on Monday afternoon and has been doing really, really well. Jack will be 4-years-old in April, and is a very handsome brindle boy who currently weighs in the neighborhood of 71 pounds. He's a little on the skinny side now and will probably be around 82 pounds (give or take a pound) when he fills out a bit.
Jack is curious, intelligent, loving, and has proven a quick learner so far. Housebreaking is going very well and he's already been up and down several sets of stairs without a hitch. He's fairly good on the leash already, and we expect him to improve even more with more practice.
Jack is getting along well with his foster brother, Coastal, and should be fine in a household with other dogs. How well he'll do with cats is still up in the air. He got excited and aggressive with a neighborhood cat while on a walk, and we've yet to introduce him to the household cat, so the jury's still out on this aspect. Obviously we'll update after some more interaction has occurred .
Jack has settled into an easy routine here already and is sleeping, eating and behaving very well. He's very fond of our daughters (ages 8 and 11) and should be fine in a household with kids in this age range.
Jack kennels up with no problem when needed, and hasn't shown any separation anxiety issues when left alone.
Between Jack's mellow and inquisitive nature, and his loving and easygoing personality, all of the early indications suggest that he's going to make a very fine companion.