Old dog doesn't care for the new dog.

I adopted our small Scottish Terrier mix (Bo) from the shelter two years ago, he was found as a stray in company of a Labrador. Bo has always done great with other dogs and always eager to play when we have gone to dog parks or while he was in obedience school.
I thought he wouldn't mind when I brought home a 3 year old Labrador (Boyd) over the weekend.
I don't know if it's a dominance thing or maybe because the new dog is in his territory, but Bo has been somewhat aggressive towards Boyd.
He barks, growls and snaps at him, although Boyd outweighs him by 60lbs. Bo gets really upset when Boyd and I are playing or when Boyd comes to me for scratches. Bo acts like he is protecting us.
Boyd on the other hand ignores Bo, when Bo is growling at him, Boyd ignores him and walks away. Bo tried to jump on Boyd's back on the first day and he just walked off, which I think made Bo mad.
Boyd was raised with a Miniature Pinscher, who always hated him and would constantly bite at him, so unfortunately I think Boyd is used to being picked on.

I've never been in this situation before and I don't know which steps I need to take. I would appreciate some advice before someone gets hurt. So far I've just been separating them when Bo gets out of control. I'm not showing any favoritism towards one or the other and Bo is not getting any less attention than he previously did.

Both boys are neutered and the same age.