Hi and welcome to Pet Talk. You might want to rub them with towels so that they can smell each others scent. It can take a long time for cats to feel comfortable with each other or even to like each other. Some cats will only learn to tolerate each other. Slow introductions are the best way to go. Also be sure to give Howie a lot of love and attention so he doesn't feel jealous of his new brother. Hissing and growling is normal cat behavior towards a new addition so I wouldn't be worried unless there is bloodshed. Hopefully Howie will stop running outside soon and will accept his new brother. Good luck and please keep us updated.
BTW my cats Storm and Cirrus still don't like each other and I've had Cirrus now for a little over 5 months now. I think they'll just learn to tolerate each other.