Anytime Sapphire looked stressed or sad, I would pick her up and hold her in my lap... then, looking in her eyes, I told her how much I loved her, and that she would always be special to me.
Now I know how this sounds, but somehow she began to understand (after about the 3rd time), I know she doesn't understand the language except for maybe a word here and there, but maybe she picked up on my body language, and I did make a point to single her out, which she noticed.
They were all around my chair, and I reached down and picked her up... I could see the look on her face, like 'You picked me! Yay!'. After a few days, I could see her changing, more confident, more like how she was before the 4th dog was added.
Now, whenever she insists on some attention, I give it to her... because to me she is very special... and will always be.
Then I go through and pick up each one and we have a personal little moment together... so funny.. like they wait their turn...
Oh and if he has a favorite thing he likes to do, take time to do it with him... wait I mean Sapphire loves to get on my bed and have a tummy rub (at least one), Finny likes up on the bed to go 'cracker dog' or zoomies, Peek just likes to be held and talked to, and Tink likes to be held, belly rub and talked to, she also loves to play with toys, so we do a fetch session.
So set him on your lap, and tell him your're sorry you hurt his feelings... you only wanted him to have a playmate and friend, and that you still love him just as much, if not more than before. And give him a kiss on his cheek.
Did you notice that? You don't split your love between the two, it's like your heart gets bigger and you make more love for the baby..