How can I get my dogs to be comfortable with each other?

Hi! I’m trying to figure out how to get my two dogs to get along. One is a chihuahua who’s much older than my frenchie. The chihuahua is probably around 6-8 years old, though I’m not sure. He can be a grumpy grandpa sometimes, but the newer dog is a puppy, about 3-4 months old, so he’s still learning. The main problem is that my chihuahua can’t see the puppy without trying to bite or bark. The frenchie wants to play with him, but I think it might be mostly behavioral issues with my chihuahua and we’ve tried everything even slowly introducing them but nothing. I’d love to hear any advice!