2 year old dog and puppy

We have a new puppy and my existing dog was fine at the start of the greeting. We met outside etc so it was neutral territory, and he had a sniff and seemed excited and fine. He doesn’t like dogs on the tv or on walks so we have been extra cautious bringing another dog in. He has lived with another dog but we lost her recently and he’s been lost without her. I’ve got a trainer helping with the introduction but it seems to be whenever the puppy moves, he barks like crazy. I feel like he’s scared? We’ve been following advice, rewarding positive behaviour etc (she only came home today!) and she has a room he doesn’t bother with usually. We have a pen around her crate, and a stair gate separating him from that room, and he can’t see her from the stair gate.

I’m just wondering if anyone knows exactly how slow I should be taking this?

Tonight the older dog will be sleeping with me in my room, which he has done ever since the other dog passed ☹️ he used to sleep with her all the time for the past two years. The puppy will be in the living room with my husband and the door will be closed.

Thanks for any advice.